Abstract:     Since the beginning time of reform and opening up, China′s economy has been growing at a high speed for a long time. However, behind the rapid economic growth, it has also brought serious problems such as high energy consumption, high emission and high pollution to China. Under this circumstance, achieving green development is an important driving force to promote China′s ecological civilization construction process and maintain the sustainable development of China′s economy and society, which is more in line with the urgent requirement of building a well-off society in China. At the same time, China has been implementing the innovative city pilot policy since 2008. Along with the implementation of the innovative city pilot policy, the government is required to increase financial expenditures for the smooth development of innovative activities on the one hand, and further promote the development of technological innovation in cities on the other hand. Therefore, in the context of China′s innovative city pilot policy, this study tries to explore the impact of innovative city construction on urban green development, and through which mediated transmission mechanism the impact is generated.
    The Yangtze River Delta region is one of the most economically developed regions in China. However, due to the long-standing influence of the rough development model and the natural ecology of Yangtze River, the environmental pollution in the Yangtze River Delta urban agglomeration is relatively serious and the overall level of eco-efficiency is low. In addition, with the support of national policies, the Yangtze River Delta urban agglomeration is also the region with the most intensive implementation of innovative city pilot policy in China. In view of this, this paper uses the panel data of 24 cities from the year of 2005 to 2019. Then this paper measures the urban green development performance by using the entropy-weighted TOPSIS method and constructing an evaluation index system based on the four-system framework model including economic development, social progress, ecological environment, and market mechanism. In addition, this paper treats the innovative city pilot policy as a quasi-natural experiment and adopts the spatial difference-in-difference model to study its influence mechanism on the green development performance.
   The conclusions of this study are as follows: (1) The scores of overall urban green development performance and subsystem performance are not high and vary among cities in the Yangtze River Delta urban agglomeration and within the three provinces and one city region, and Anhui Province is in a disadvantageous position in the green development process. This result also reflects that there are still limitations in the green development of cities in the integration process of the Yangtze River Delta urban agglomeration, and the green development performance is spatially heterogeneous among cities. (2) The innovative city pilot policy has a promoting effect on the urban green development performance, and it also has a positively spatial spillover effect on the green development performance of neighboring cities. In other words, the innovative city pilot policy not only has a positive impact on the urban green development performance of the region, but also generates a "policy dividend" on the urban green development performance of the neighboring regions. (3) By using the parallel trend test, counterfactual check, and changing the method to measure the dependent variable, namely urban green development performance, this paper checks the robustness of the benchmark regression results. And, the results also further verify the reliability of the conclusions of this paper. (4) The innovative city pilot policy has a significantly positive impact on both green technology innovation and industrial structure upgrading of the Yangtze River Delta urban agglomeration. In addition, green technology innovation and industrial structure upgrading play multiple mediating effects in the influence mechanism of the innovative city pilot policy on the urban green development performance, and the proportion of mediating effects is higher for green technology innovation than industrial structure upgrading.

Key words:  innovative city construction, green development, green technological innovation, industrial structure upgrading, spatial difference-in-difference model