The process of earthquake incubation is accompanied by tectonic movement， material migration and density changes， which will cause changes in the Earth's gravity field. Repeated observations of flowing gravity may capture precursor information related to earthquake incubation， thereby providing important basis for mid- and short-term earthquake prediction. Based on the relative gravity observation data of the North-South seismic belt from 2017 to 2020， this paper obtains the spatio-temporal changes of the regional gravity field in southeastern Gansu， and analyzes the dynamic changes of the regional gravity field and its relationship with the occurrence of the Xiahe M_S 5.7 earthquake in Gansu in 2019. The results show that： \mathrm{①} The contour of the gravity anomaly change in the observation area before the earthquake is basically the same as the strike of the Lintan-Tanchang fault， and the four-quadrant distribution characteristics appear near the epicenter. The Xiahe M_S 5.7 earthquake occurred in the high gradient zone and near the center of the four-quadrant. \mathrm{②} The regional gravity field before and after the earthquake experienced a spatiotemporal evolution process of "regional gravity anomaly-four-quadrant distribution characteristics-reverse change earthquake". \mathrm{③} The occurrence of the Jiuzhaigou earthquake may cause the adjustment of the stress field in the nearby fault area， thus accelerating the occurrence of the Xiahe earthquake. \mathrm{④} The time-varying regional gravity field corresponds well to the Lintan-Tanchang fault in space. Comprehensive analysis can be considered that the seismogenic fault of this earthquake is the Lintan-Tanchang fault.