This paper elaborates on the mechanism of the impact of the opening of high-speed rail on the development of urban agglomerations. Taking the data of 92 prefecture-level cities in China"s five major urban agglomerations from 2005 to 2018 as samples, and starting from the perspective of urban agglomerations, this paper investigates the impact of the opening of high-speed rail on the pattern of economic development and its mechanism by constructing DID estimation model. The research finds that: from the overall perspective of the five major urban agglomerations, the opening of high-speed railway has improved the economic growth level of the five major urban agglomerations, and narrowed the economic gap of urban agglomerations; The results of urban agglomeration show that the opening of high-speed railway has a positive effect on the economic growth of each urban agglomeration, and significantly reduces the economic gap between the Beijing-Tianjin-Hebei, Yangtze River Delta and Pearl River Delta urban agglomeration, which is conducive to the realization of economic integration. The concentration of talents, capital and technology innovation play an intermediary role in the process of the opening of high-speed railway affecting the pattern of economic development.