Abstract:     China has always attached great importance to technological innovation and it has even been upgraded to a strategic height that related to national competitiveness. Since the reform and opening up, R&D investment has increased year by year, but there is still a gap between R&D investment intensity and innovation quality compared with developed countries. As the main body of R&D innovation, enterprises are the main driving force of the improvement of China′s innovation level, and its contribution to R&D expenditures account as high as 77.4%. However, there also exist problems like insufficient innovation motivation and low quality of enterprises R&D innovation. To enhance the innovation level, we must pay attention to both quantity and quality, and we must give full play to the enthusiasm of enterprises. The inadequate of enterprises R&D innovation capabilities are mainly susceptible to two factors that is the public goods characteristics of innovation itself, and the failure of financial markets. The former has a significant "spillover effect" due to the "public goods" characteristics of innovation, resulting in private innovation level below the overall level of society. Scholars, represented by Pigou, believe that taxation can solve the problem of spillover problems, and tax incentives are increasingly recognized since their sufficient market orientation and tax statutory advantages, and gradually become the main fiscal and tax incentive method. The latter, "financial market failure", is mainly manifested in the susceptible of innovation to capital restrictions. In the perfect capital market, internal and external financing of enterprises are completely substitutable, yet problems, such as information asymmetry and principal-agent problem, make the two irreplaceable and lead to financing constraints. Therefore, it is necessary to integrate financing constraints, tax incentives and enterprises R&D innovation into a framework for research.
    The existing literature mainly focuses on the examination of the impact of tax incentives on enterprises R&D innovation, or the impact of financing constraints on enterprises R&D innovation, but few have studied the impact of tax incentives on enterprises R&D innovation from the perspective of financing constraints. In the process of economic development, the lag of financial development or the formation of financial repressive systems is common phenomenon, which is more common in developing countries. If policy makers in developing countries only use incentive method such as tax incentives and government subsidies to stimulate enterprises R&D investment, and ignore the obstacles to financing channels in enterprises R&D innovation, it may lead to the failure of government incentives policies. The possible contributions of this paper are: First, the paper investigate the impact of tax incentives on enterprises R&D innovation from the perspective of financing constraints, and further explore the transmission mechanism of tax incentives on enterprises R&D innovation from the aspects of both internal financing constraints and external financing constraints. Second, we examine the effects of two types of tax incentives, tax rate incentives and tax base incentives, on enterprises R&D innovation, which provides theoretical and empirical evidence for the optimization and reform of corporate income tax incentives. Third, we study the enterprises R&D innovation from two dimensions, namely the quantity and quality of R&D innovation. Efficient tax incentive can not only increase the intensity of R&D investment, but also guide enterprises to improve the quality of R&D innovation. Only high-quality patents can promote the development of enterprises, thus it is of more practical significance to study the issue from both dimensions of quantity and quality.
    This paper uses the panel data of Shanghai and Shenzhen A-share listed companies over the period 2010-2017, it relies on the Euler equation investment model to build an econometric model and adopt two-way fixed effects model, dynamic regression model, negative binomial regression model and other methods. First, from the perspective of financing constraints, the effects of tax rate incentives and tax base incentives on the quantity and quality of enterprises R&D innovations are studied, and their results are tested for robustness. Second, it further answers such questions as whether enterprises face financing constraints and whether tax incentives and financing constraints will lead to R&D manipulation. Finally, the transmission mechanism of tax incentives on enterprises R&D innovation under the financing constraints is studied. The transmission mechanism of tax incentives on enterprises R&D innovation is mainly studied from two channels: that is, internal financing channel and external bank credit financing channel.
    The research results in this paper show that: (1) Tax incentives have positive promotion effect on the R&D investment intensity, and the promotion effect of tax base incentives is greater than tax rate incentives, but further research the impact of tax incentives on R&D manipulation discover that tax rate incentive may lead to manipulation of enterprises R&D. (2) The promotion effect of tax incentives on the quality of R&D innovation is insufficient, especially when the positive promotion effect of tax base incentives on the quality of enterprises R&D innovation has not been formed. (3) Chinese enterprises face relatively strict financing constraints. The impact of financing constraints on the R&D investment intensity is significantly negative, and they are more susceptible to internal financing constraints. Tax incentives can alleviate financing constraints, which in turn promotes enterprises R&D innovation. Among them, tax base incentives are mainly helpful to alleviate internal financing constraints, and tax rate incentives are more helpful to ease external financing constraints. 
    The enlightenment obtained at the end of the paper is that: (1) Different tax incentives should make good use of each advantage. In regard to tax base incentives, the scope of R&D additional deduction should be further expanded, and the deduction ratio should be increased; and tax credit should be introduced. With respect to tax rate incentives, the overall tax burden of the enterprise should continue to be reduced, and the negative impact of tax incentives based on qualification recognition should be minimized. Attention should also be paid to the possible adverse impact of tax rate and tax base policies mix. (2) Tax incentive policies should serve for high-quality innovation, for example, it can guide enterprises to pay more attention to the quality of R&D innovation through precise and differentiated tax incentives and high-quality tax collection and management. (3) Although tax incentives can alleviate the problem of enterprises financing constraints, the problem still need to be mitigated through channels like financial reforms, financial system improvement and vigorously development of financial markets, which can ensure the effectiveness of policy transmission channels.

Key words: tax incentive, enterprises R&D innovation, financing constraint