In this paper, the gravity changes of one year scale before and after the Minxian—Zhangxian M6.6 earthquake on July 22, 2013 are analyzed using the mobile gravity observation data from 2010 to 2014 in southeast Gansu network, and then the relationship between the gravity and b-value anomalies in southeast Gansu and the Minxian—Zhangxian M6.6 earthquake is discussed considering the b-value image and global navigation satellite systems result. The results indicate the following: (1) The gravity anomaly in the study area underwent a spatiotemporal evolution from regional negative gravity change to constant negative change, positive change of turn and increase, earthquake occurrence, and then recovery after the earthquake; (2) before and after the Minxian—Zhangxian M6.6 earthquake, the b-value curve shows characteristics of low value-high value-reduction and shock; (3) the Minxian—Zhangxian M6.6 earthquake occurred in a high gradient zone of gravity change near the "0" contour line and the low b-value region.