By using mobile gravity observation data in the Tibet Plateau during 2015-2020,the gravity variation characteristics before the Biru 6.1 earthquake on March 19,2021 was studied. The results show that:① the high gradient zone along North-South direction,which is across the epicenter and in consistent with the typical gravity anomaly characteristics,has been observed in the gravity variation images of about 3 years before the earthquake. This indicates that the sign of gravity anomaly caused by the earthquake preparation has been detected. ② The observed gravity variations are basically consistent with the gravity variations caused by the dislocation of a normal fault. Considering the focal mechanism of this earthquake is mainly normal fault,the East-West stretching movement of Qiangtang block may be the background of this earthquake occurrence.