Based on the sixteen gravity sections chosen, the author has calculated anomalous value of gravity in the depths of the earth. Basic form and degree of hidden depth in the earth of the Moho discontinuity have been got. The thickness of the crust in Ningxia area is 1-5KM greater than that of Shanxi area and 6 KM less than that of Gansu area. There are three segs and one upheaval of the Moho discontinuity in this area. They are the Yinchuan-Pengluo seg, the Wuzhong-Majiatan seg, Haiyuan-Lijunpo seg and the Zhongning-Zhongwei upheaval. On the south of Ningxia area they are generally bounded by the Daluoshan Mountain, the Xiaoluoshan Mountain, Yuwang and Jingyuan. Curst is getting thiner and thiner on the east of the bound and thicker and thicker on the west of the bound. Historical strong earthquakes just occurred on the stair belt between the segs and the upheaval of Moho dicontinuity in Ningxia area.