Abstract: Based on the observation data of relative gravity and absolute gravity in the southern section of the South-North Seismic Belt since 2015, we have systematically analyzed the dynamic change of regional gravity field and its relationship with the occurrence of Yangbi M_S6.4 earthquake on May 21, 2021. The results show that: ① The change of regional gravity field is closely related to the Weixi-Qiaohou fault, which reflects that the fault activity caused the change of surface gravity field from 2015 to 2021; ② The gravity field change related to the preparation of Yangbi earthquake has experienced the evolution process of “steady state-regional gravity anomaly-local gravity anomaly-four-quadrant distribution-large area positive anomaly-earthquake occurring in the reverse change process”; ③ The images of cumulative change and differential change of gravity field show that there were significant gravity anomalies in the two years before Yangbi earthquake, and the earthquake occurred in the gradient zone of gravity change, the center of the four-quadrants and near the zero contour curve; ④ The dynamic evolution image of gravity field can well reflect the medium-term precursory phenomena during the preparation of Yangbi M_S6.4 earthquake, and based on this phenomenon, a certain degree of medium-term prediction was made before Yangbi M_S6.4 earthquake, especially the determination of strong earthquake location.