From January 15 to January 27 in 2015，a ML3.3 earthquake swarm happened in Jiexiu, Shanxi. The results of accurate location showed that seismic sources were small and the depth of hypocenter was shallow. In ferms of the characteristics of corner frequency and apparent stress，the activity of earthquake swarm was similar with reservoir induced earthquakes. Considering water's participation in seismic process，this paper discussed the causes for the Jiexiu earthquake swarm by the analysis of tectonic environment，seismic source parameters，fluctuation of Jiexiu well and the regional stress field in different periods. All the evidences showed that fracture，groundwater infiltration and diffusion which weakened rock mass's strength were the main reasons for Jiexiu, Shanxi ML3.3 earthquake swarm. At the same time，the seepage of groundwater made seismic wave attenuate sharply，and the corner frequency，as well as apparent stress，was lower than other area in Shanxi，which had same magnitude level.