The relationships of the seasonal variation in earth rotation with the medium-small earthquakes around the epicenter before the M_S8.0 Sichuan Wenchuan earthquake on May 12,2008, the M_S7.1 Qinghai Yushu earthquake on April 14,2010 and the other fourteen M≥7.0 earthquakes have been studied in this paper. From this research we find that the medium-small earthquakes in the regions of aftershocks within several years before the strong earthquakes often occur in only one period of acceleration periods and deceleration periods of the seasonal variation in earth rotation. This research will benefit for us to further understanding the relationship between earthquakes and earth rotation, and there is a certain guiding significance for earthquake prediction.