Abstract: On April 15, 2015,a M_S5.8 earthquake occurred in Inner Mongolia's Alxa Left Banner. 60 focal mechanism solutions are collected around the epicenter. Stress field of the epicenter area is inverted by using MSATSI software. The results show that this earthquake occurred in the regime of strike-slip system. The inhomogeneity of stress field is not spatially obvious. The trend of the principal compression stress is N42±7°E and that of the principal tensional stress is N58±12.6°W. The R value is ~0.4. Compared with the results of previous studies, the directions of compressional and tensional stress axes are more consistent with the P, T axes of the focal mechanism of this earthquake. In order to understanding the relationship of mainshock and aftershocks of this earthquake sequence, we calculated the static Coulomb rupture stress changes produced by this earthquake. Results show that coulomb stress changes at the epicenter of M4.0 and M3.1 aftershocks are 0.043 MPa and 0.023 MPa, respectively, which show the triggering effect on aftershocks of this earthquake.