The Xiaowan reservoir is the leading reservoir of cascade hydropower station in the middle and lower reaches of the Lancang river,lying in an area of active tectonics. In this paper,we used both absolute arrival time and relative arrival time of 11249 earthquakes around the Xiaowan reservoir during two periods after impounding:December 16,2008 to June 30,2011 and July 1,2011 to December 31,2016,and with the double-difference seismic tomography method. Finally,we obtained the P-wave velocity structure and relocation of earthquakes in the Xiaowan reservoir area. We found that the increased earthquakes in the Heihuijiang river and backwater section of the Xiaowan reservoir are related to the reservoir impoundment. The pore pressure changes due to the infiltration of the water body,and the pore pressure changes to the deeper part with the change of time,which leads to the change of medium and the decrease of P-wave velocity. There are more earthquakes in the Lancang river after the return of water storage to the Baoshan section of the Lancang river. It causes the P-wave velocity to drop. Reservoir water infiltration is the main control factor. And the aftershock sequence of the Channing M_S5.1 earthquake on October 30,2015 is tectonic earthquake that is related to the water storage. At the same time,there is an activity which is not related to the water storage. The Shidian region always has earthquakes,which are tectonic earthquakes and have nothing to do with reservoir storage.