Abstract: This paper comprehensively analyzed the anomalous variation in gravity measured before moderately strong earthquakes occurred in Ningxia region since 1980 based on the data of groundwater, vertical deformation, regional stress field and geological structure. The results show that the variation in gravity has no clear relation with the variation in ground-water level and is not due to vertical crustal deformation either Its main cause may be that it was caused by the variation in material densisy in the active fault zone and a special position near it under a strong action of the NE horizontal compressive stress field. Meanwhile a simulative calculation was made for the strlke-slip of Haiyuan large fault and the result supports the above idea.