Abstract: Using the panel data of China's 282 cities from 2003 to 2014,the relationship between city size and local environmental pollution was empirically investigated. By constructing three complementary measurement indicators of city size,using System GMM estimation method it was found that the expansion of city size has a significant negative effect on local environmental pollution. Through the analysis of the heterogeneity of quantitative and regional grouping,it was found that the expansion of the scale of big cities and cities in North China has a more significant effect on reducing environmental pollution. In the context of accelerating urbanization and managing environmental pollution in China, it is advisable to control the deteriorating regional environmental pollution by encouraging the development of big cities.