Abstract: There occurred a M_S6.4 earthquake in Menyuan county, Qinghai province, on Jan 21, 2016. The focal mechanism solutions are collected around the epicenter. Stress field of the epicenter area is inverted by using MSATSI software. The results show that the maximum principal compressive stress axis gradually changed from NE direction in west region to NEE direction in east region with small dip angle, nearly perpendicular to the active faults with azimuth of NW and NWW direction. The direction of extensional stress axis has strong inhomogeneity, but the dip angle is consistently large in study region. The rotation of the maximum principal stress was mainly caused by the Tibetan plate moving in NE direction, blocked by the hard Alashan block in the north, and forced to extrude in SE direction, which forming a left lateral strike slip and thrust regime. The P and T axes of the focal mechanism of this earthquake are more consistent with the directions of compression and tension stress axes respectively, which indicates that the earthquake occurred in the thrust and left-lateral strike slip tectonic setting of the northeastern margin of the Tibetan plateau.