By comparing and analysing the characteristics of gravity fields on two sides of a typical active fracture, the author finds that the massifs where Bougucr gravity anomalies are regionally negative (or relatively negative) always move towards the equator, while the massifs where the Bouguer gravity anomalies are positive (or relatively positive) move towards the poles. The map of Bouguer gravity anomaly portrays the density distribution of the massifs. The massif with positive gravity anomaly has a high density and that with negative gravity anomaly has a low density. According to the theoretical analysis, the change in density of the massif will lead to a change in stress conditions, which causes the loss of stability of the massif and put massif in motion. The magmatic activity is an important factor for the density change.