The Yanhe M_S4.9 earthquake occurred in the north-south direction of Fenggang fold deformation area of the upper Yangtze block,where very few earthquakes had been recorded in the past. Through seismic geological background analysis,earthquake damage investigation,focal mechanism solution,fault investigation and water level change in the reservoir area,we obtained the main understandings as follows. Due to the shallow focal depth and poor seismic performance of old self built houses in the disaster area,the direct economic losses of the earthquake were serious. The focal mechanism of the Yanhe earthquake is as that:nodal plane I was strike=61°,dip=35°,rake=135°,and nodal plane II was strike=190°,dip=66°,rake=63°,which suggests a NE strike,thrust and translation. Combined with the trend of isoseismal line and the properties of main faults at the epicenter,we believe that the NE Yanhe Fault is the seismogenic structure of the main earthquake of this earthquake. it is further inferred that the earthquake was caused by reservoir induced fault activation.