This article starts from the perspective of spatial economics and discusses how urban agglomeration economy influence location of new firms. Theoretical research reveals that specialization and diversity externalty both promote enterprise entry. This theoretical inference is supported by empirical evidence based on the matching of corporate micro-data and city data. From 2001 to 2008, China specialized agglomeration economy and diversified agglomeration economy had a significant positive impact on the creation of new enterprises. In addtion, the degree of administrative monopoly of the government and its preference for state-owned enterprises will weaken the positive externality of agglomeration on the creation of enterprises. After controlling for the missing variables and using instrumental variable to control endogeneity problems, the results are still robust. On this basis, the further analysis of its mechanism show that input and output sharing, labor pooling, and technology spillover effects are all important channels for agglomeration of externalities to promote the establishment of new firms. Similarly, government intervention has a negative moderating effecton the above three mechanisms.