Using continuous and complete data of small earthquakes recorded by Xiaowan Reservoir Seismic Network, the authors calculate the variation of seismic wave velocity ratio for one year before and after water storage of Xiaowan reservoir in earthquake cluster area, and analyze significant anomaly of wave velocity ratio before M 3.0 earthquakes. The results indicate that earthquake swarm area distributes on Changning and Fengqing counties of Lancang River Fault in Xiaowan reservoir and Niujie Township in Weishan County in Heihuijiang reservoir. The wave velocity ratio of Fengqing area in back reservoir is lower than that of Changning area in the middle of the reservoir, and it distributes non-uniform in space. The water storage of reservoir has certain influence on the underground medium in Fengqing area. The wave velocity ratio before M 3.0 earthquakes appears lower value anomaly before water storage.