Abstract: In this paper, data of mobile gravity survey and the observation precision within ten years before and after the 1976 Tangshan earthquake were investigated. It was proved that there existed a trend of anomalous change in the epicentral region before and after the earthquake through the noise-depression experiment and reliability analysis. We analysed the physical factors which caused the gravity variation.  Bandpass filtering method in the time domain was adopted to preprocess the data. A fitting curved surface to the values of gravity survey was obtained. We also analysed the values of anomalous change at equally spaced points in the region with the aid of the moving window average method to filter out the gravitational effect caused by shallow short wavelength density changes. Restraint inversion was done according to the remainder of the abnormal gravity field and it was found that deformation in rock formation in the vicinity of the Moho surface in this area might have appeared before, during and after the earthquake; especially, the deformation amplitude might be rather large several years proior to the earthquake. It is shown that the deformation of the rock formation near tie Moho surface may be a significant cause of stress concentration in the crust and the seismogenesis of the Tangshan earthquake.