Using data of mobile gravity measurement, variation features of gravity field in south area of northern segment of the North-south seismic belt is analysed. The results show that using 105°E as a boundary, gravity field variation of the east side of the boundary is different from that of the west side of the boundary, reflecting there is a NS active tectonic zone in deep crust along 105°E. In Gansu province, three M_S ≥ 5 earthquakes occurred since 1990. Before the three earthquakes, anomalies of gravity differences of Anmencun, Lanzhou and Shuping, Yongdeng observation sites are obvious.