The earthquake swarm activity in the Shanxi Reservoir, Zhejiang Province, in September 2014 has attracted widespread attention. However, the understanding of its tectonic stress environment remains controversial. Therefore, using earthquake catalogs and phase observation reports for Shanxi Reservoir from September 2014 to April 2015, this study calculated the b-value of the earthquake swarm, performed precise relocation using the double-difference relocation method, and analyzed the seismogenic structure and genetic mechanism of the swarm activity by combining focal mechanism solutions and stress field inversion results. This research indicates the following: (1) The spatial distribution of relocated hypocenters and the statistical analysis of nodal planes from focal mechanism solutions both revealed distinct NW—SE-trending high-angle tectonic features, consistent with the structural characteristics of the third branch fault of the Shuangxi—Jiaoxiyang fault. (2) The b-value of the earthquake swarm was 0.76, reflecting reservoir-induced seismic characteristics with small magnitude and high frequency. Most focal mechanism solutions of M_L 3.0 earthquakes in the swarm were of the strike-slip type, consistent with the tectonic features of the principal stress axes of the regional stress field obtained from small earthquake polarity inversion. Accordingly, this study concludes that the swarm represents a tectonic-reservoir-induced earthquake, resulting from mechanical instability of the third branch fault of the Shuangxi—Jiaoxiyang fault under the control of the regional stress field.