Abstract: In this paper, we propose a method in the analysis of city system based on triangle transition, and empirically analyze the spatial structure of Chinese cities. This triangle transition maps the geographical location of the target city and its three neighboring cities of a certain condition to a standard equilateral triangle space. Through analyzing the position distribution of the cities in the mapping triangle, we observe the gathering effect between the neighboring cities with similar GDP, while remarkable centrifugal effect between the neighboring cities with large difference on GDP. The former implies the competitive trends, the latter shows the collaborative relationship between cities. This transition from competition to collaboration occurs at threshold 1.2 on the ratio of the two cities' GDP. Furthermore, based on this method, we propose an approach to identify economic regions, and analyze the regional economic structure of the mainland of China. These methods based on triangle transition provide a novel viewpoint to understand cities' economic competition and collaboration, as well as the trends of regional economy development.