Abstract: Based on the analysis of the general characteristics of the durations of the aftershock sequences of 39 relatively larger earthquakes in China in recent years and their relation with the stress drops of the main shocks and the negative anomaly of the seismic wave velocity ratios after the main shocks, a tentative idea has been advanced that the aftershock activity of an earthquake can be separated into two stages by the transition point of the seismic wave velocity from decreasing to increasing after the main shock, and a discussion made of the characteristics of the aftershock activity in each of the two stages.