This paper considers that urban flow intensity describes the contacts among cities, and it is the quantitative index of urban service function. From the strong correlation between urban flow intensity and centrality index, The paper indicates the tight relations based on the demonstration of three provinces. With the case study of Shandong Peninsula, Zhongyuan and Guanzhong urban agglomerations, this article analyses the structures of urban flow intensity, discusses urban agglomeration evolution mechanism from the urban flow angle, and discovers the rule of urban agglomeration evolution from the west to the east in China. Also, it reveals the evolving courses of urban agglomeration space structures from the simple single center to complicated polycentficness. According to the conditions of urban flow magnitude and its industry distribution, the paper analyses the secondary and tertiary industry urban flow intensity and its structures of urban agglomerations as well as its core cities, and draws out its external service character. The development level of agglomerations has a close relation with the second industry urban flow intensity. With the development of the second industry, agglomeration economic strength will be stronger, its status of external service functions will be higher. Compared with the eastern agglomerations, the functions of central city in western agglomeration increase rapidly. But the capability of centralization and radiation in midst agglomeration is not strong, its external service functions are concentrated on basic service departments.
From the urban flow growth of central city in the three agglomerations, the paper shows the regional central status of Zhengzhou improves gradually. The growth of central cities' external services in Guanzhong urban agglomerations is faster than Zhongyuan urban agglomeration as well as in Shandong Peninsula agglomeration. This indicates Guanzhong and Zhengzhou agglomeration are still in the early stages of agglomeration evolution with polarization effect continuously strengthened, and Shandong Peninsula agglomeration is relatively in the mature stage with its diffusion effect and regional integrative process accelerated continuously.