China is stepping into the high-speed railway (HSR) era. The time distance gradually replaces the traditional space distance, which has become the key factor affecting the layout of the service industry. The large-scale construction and network layout of the HSR in the Yangtze River Delta have weakened the space barrier to the flow of production factors, and have made the optimal allocation of resources in a wider range. Using statistical data and train operation data in the Yangtze River megapolis from 2006 to 2015, this paper quantifies the "space-time convergence effect" caused by HSR implementation by constructing the difference-in-difference model and introducing the gravity model in economic geography, and quantitatively analyzes the impact of HSR evolving from the line into the network on urban service industry agglomeration. It arrives at the results as follows:firstly, at the early operation of HSR (2007-2011), the impact of HSR on urban service industry agglomeration in the Yangtze River megapolis is not significant; and since 2012, with the rapid development of the HSR network in the Yangtze River megapolis, HSR has played a significant role in promoting service industry agglomeration, and the intensity of the impact has gradually increased; secondly, after the implementation of the HSR network, the location of the urban market along the HSR line has been improved, however, there are obvious differences for the improvement in the time and space. The unbalanced time-space convergence effect caused by the HSR network in the Yangtze River megapolis results in the reconstruction of urban market location level along the HSR line, and has exacerbated the imbalance of the location conditions of big cities and small cities; thirdly, the spatial effect caused by HSR increases by 1% each year leads to the rise in the employment density of urban service industry along the HSR line by about 0.3%~0.4%.