Abstract:    Along with the rapid economic development, China has become the second largest economy in the world. In order to further promote the leapfrog development of the economy, the field of high-tech industry will certainly become one of the important breakthrough points for economy development. Over the years, China has always attached great importance to the development of high-tech industries and has issued many incentive policies. Among them, the national high-tech zones policy is one of the earliest and most important policies for the development of high-tech industries in China. As an important platform for the development of high-tech industries, national high-tech zones provide many supports for enterprises in the zones, including tax incentives, talent subsidies, bank credits, etc. National high-tech zones have realized the clustering of high-tech industries, accelerated the speed of knowledge and technology dissemination among high-tech enterprises, and facilitated the realization of innovative development of enterprises. The national high-tech zone policy has been implemented for more than thirty years, and by now, more than half of the prefecture level cities have national high-tech zones. What kind of realistic role high-tech zones play in the development of high-tech industries in each city has become a question worth exploring.
   In this paper, we use panel data of 284 prefecture-level cities from 2003-2018 to analyze the impact of national high-tech zones on the development of high-tech industries through the difference-in-differences method. The results show that the national high-tech zone policy significantly promotes the development of high-tech industries in the cities where they are located. China is a vast country with many cities, and different cities and regions differ greatly in economy, social and culture, which may lead to different effects of high-tech zones on the development of high-tech industries in different cities. In addition, national high-tech zones are established in batches, and the years of establishment may also be an important factor affecting the effect of high-tech zones. 
    In order to further explore the difference of the influence of high-tech zones with different conditions on high-tech industries, this paper conducts three heterogeneity tests according to city rank, region and growth cycle. From the perspective of cities, the role of national high-tech zones on the development of high-tech industries varies depending on the development level of the city where they are located, showing the law of "diminishing marginal utility". With the improvement of the city′s comprehensive level, the significance of high-tech zones on the development of high-tech industries shows an inverted "U" shape of increasing firstly and then weakening. High-tech zones in second-tier cities and below have significant effects on high-tech industry development, among which, the policy effect of high-tech zones in third-tier cities is the most significant, followed by fourth and fifth-tier cities, the effect of high-tech zones in second-tier cities on high-tech industry is the weakest, while high-tech zones in first-tier cities have no significant effect on high-tech industry development.

    From the perspective of region, the economic development in the western region falls behind, lacking the basic conditions required for the development of high-tech industries, and the policy effect of high-tech zones in cities in this region is not significant. The high-tech industry in the cities in the central region has achieved certain development and still has more room for development, and the establishment of high-tech zones can effectively enhance the development of high-tech industry in the cities in this region. The high-tech industries in cities in the eastern region started early and developed relatively maturely, and the establishment of high-tech zones did not have a significant effect on the development of high-tech industries in the eastern cities. From the perspective of the life cycle in which the high-tech zones are located, the high-tech zones in the start-up and development stages effectively promote the development of high-tech industries, while the policy effect of high-tech zones in the mature stage is not significant.

    Based on the empirical results, the following suggestions are made: (1) Give full play to the guiding role of national high-tech zone policy on the development of high-tech industry. Combined with the national development plan, the government supports the development of target zones through the means of high-tech zone site selection and preferential policies to some high-tech zones to enhance the balance of high-tech industry development among different regions. (2) Strengthen the quality construction of national high-tech zones according to the local conditions and enhance the technology innovation ability of high-tech enterprises. (3) The development of high-tech industry should not only rely on the policy of high-tech zone, but also consider various factors, and rely on more factors, including continuously expanding the opening to the outside world, actively introducing foreign investment and advanced technology, while continuously training high-end talents and encouraging independent research to promote the development of high-tech industry to a higher level.

Key words: national high-tech zone, high-tech industry, difference-in-differences (DID), China