Abstract:     China has made great achievements in its economy after 40 years of rapid development. However, the extensive development model adopted by China has led to excessive consumption of natural resources and continuous deterioration of the ecological environment, which seriously affects the economic and social sustainable development of China. Chinese government has attached great importance to the construction of ecological civilization and environmental protection since 2012, proposing to "implement the strictest possible systems for environmental protection, and develop eco-friendly growth models and ways of life", which helped formulate a series of environmental protection policies, and improved the legal system of environmental protection in China. Under the strong advocacy of the Chinese government, enterprises have been carrying out green technology innovation and transforming production modes, to achieve green development. However, due to the objective existence of the differences in economic development and industrial structure among eastern, central and western regions, different environmental policies and measures will inevitably exert different impacts on green innovation activities of enterprises.
   Therefore, based on the industrial provincial panel data of China from 2007 to 2016, this paper uses the systematic GMM model to analyze the impact of diverse environmental regulation tools, including direct regulation, economic incentives and public participation, on green technology innovation in various regions of China, along with its lag effects in the case of regional economic development differences. Moreover, the reasonable environmental regulation interval in different regions is explored in this paper with the dynamic threshold model. This paper draws conclusions as follows: 
   First, different environmental regulation tools exert different lag effects on green technology innovation. In eastern region, economic incentives at both current phase and lag one phase can promote green technology innovation, but the effect caused by direct regulation and public participation on green technology innovation is not significant. With regard to the central and western regions, the direct regulation at the lag one phase can promote the innovation of green technology, while at the current phase, the direct regulation has no significant impact on the innovation of green technology. Even more, economic incentives and public participation have little effect on green technology innovation. Besides, the impacts of the three regulatory tools on green technology innovation are not significant after lag two phase.
   Second, as for the eastern region, economic incentives exert a single threshold effect on green technology innovation. More specifically, the promoting effect of economic incentive on green technology innovation reaches the maximum when the incentive intensity is higher than the threshold value (-8.3404). In central region, the direct regulation exerts a single threshold effect on green technology innovation. Specifically, the direct regulation will significantly promote the innovation of green technology when the intensity of regulation is lower than the threshold value (-0.146). As for western regions, the direct regulation exerts a double threshold effect on the incentive effects of green technology innovation. Particularly, the incentive effect of regulation on the green technology innovation of enterprises will not be obvious when the intensity of regulation is lower than the smaller threshold value (-0.2785). Correspondingly, the effect will be significantly weakened when the intensity of regulation is higher than the larger threshold value (-0.1801). Only when the intensity of regulation is within the range between -0.2785 and -0.1801, the incentive effect of direct regulation on the green technology innovation of enterprises will be optimal.
   Based on the conclusions above, this article gives the following suggestions:
   First, the central government should formulate different environmental regulatory policies and implementation intensity based on the actual situation in different regions. It is generally acknowledged that a sound environmental regulation policy can promote the reasonable allocation of regional resources and maximize their role in the development of green technology innovation. Thus, in the eastern region, the government should focus mainly on economically-stimulated regulatory tools, and the intensity of regulation should be controlled within the optimal regulatory range of promotion, so as to maximize green technological innovation. With regard to the central and western regions, direct regulation tools should mainly be used, and environmental regulation intensity should be strengthened in the central region; specifically, as to the western region, the government should control the intensity of environmental regulation within the optimal range of promotion, give full play to the incentive effect of environmental regulation, to avoid the generation of adverse effects on green technology innovation exerted by too high or low intensity of regulation. 
    Second, a long-term mechanism of environmental management and control should be established as soon as possible in order to improve the environmental governance system. At present, government′s environmental regulation policies are majorly based on direct regulation and economic incentives, while the impact of public participation on green technology innovation is limited. And environmental regulation policies are mainly implemented by the government and enterprises, while social organizations and the public know little about this kind of public information. Therefore, the government should further improve relevant laws and regulations, give full play to the role of social organizations and the public in monitoring environmental governance, and should strengthen the incentives of public participation-based regulatory tools for green technology innovation. A government-led, enterprise-based, social organizations and the public-participation environmental governance system should be established. These measures can encourage enterprises to change from end-of-pipe control to source control, improve their green technology innovation capability and to transform as well as upgrade to green production.
    In the future, the government should further improve relevant laws and regulations, give full play to the role of social organizations and the public in monitoring environmental governance, strengthen the incentives of public participation-based regulatory tools for corporate green technology innovation, and build a government-led, enterprise-based environmental governance system jointly-participated by social organization and the public. These measures can contribute to motivating enterprises to change from end-of-pipe control to source control, improving their green technology innovation capabilities, and transforming as well as upgrading enterprises to green production.

Key words: environmental regulation tool, regional difference, green technology innovation, dynamic threshold