By using waveform data from the Pubugou reservoir seismographic network and Chengdu digital telemeter seismographic network，we found lots of small earthquakes in reservoir area are results of explosions． The research has changed the previous understanding that seismicity level is higher before the impoundments rather than after． There was one earthquake with magnitude greater than ML1. 5 within 10km of water area of the Pubugou area after impoundment: ML1. 7 on August 8，2010． In fact，earthquake occurring before or after impoundment is rare． The proportion of strike-slip earthquakes is larger than others in the two clusters which are on the west of Pubugou reservoir，and the direction of principal compressive stress of these clusters is NWW． On the east and north of reservoir the focal mechanisms take on diversity． The small earthquake activity is totally controlled by region stress field，and NWW horizontal principal comprehensive stress may help generate the strike-slip rupture of moderate and small faults with direction of NNW．