Abstract: In the paper,the crustal thickness of the north segment of the North-South Seismic Belt is inverted by means of gravity data and a single crustal model. According to the Airy isostatic hypothesis,computation of isostatic compensation is made and a map of the M surface and a 11 map of isostatic anomaly are drawn up.According to the characteristics of crustal structures,six regions of crustal structure are delineated. Preliminary discussion of the stereoscopic cross structure between Tianshui and Wenxian is made. In the light of historical data of earthquakes,the relationship between crustal stracture,isostatic anomaly and earthquakes is discussed.