Abstract: An M_S4.0 earthquake occurred in Lingwu, Yinchuan City, Ningxia Hui Autonomous Region at 20:42:54 BJT on November 18^th, 2021. It was felt by almost all people in Lingwu and its surrounding area and attracted considerable concern in the society. In this paper, the focal parameters of the earthquake are studied using Hypo2000 locating, first arrival P phase locating and gCAP method, and under current regional stress field system the relative shear stress and normal stress on the two nodal planes from the focal mechanism are also calculated. The results show that the initial rupture depth of the M_S4.0 Lingwu earthquake is 20 km, and the centroid depth is 19 km, indicating that the focal depth is relatively deep. Focal mechanism solution of gCAP method is strike 280°, dip 61°, rake 30° for the nodal plane I, and strike 174°, dip 64°, rake 147° for the nodal plane II, and the moment magnitude is M_W4.1. The full moment tensor solution shows that the double couple component of the M_S4.0 Lingwu earthquake accounts for 85.18% of the total moment tensor solution, which is a natural seismic event, and combining with the Moho structure, crustal deep-seated thermal structure and regional geological structure, the earthquake mechanism of the M_S4.0 Lingwu earthquake may be related to the upwelling of upper mantle material in Yinchuan graben. The thermal material has a certain weakening effect on the fault, and the focal area is not easy to form a large stress accumulation so that the Lingwu earthquake released in a moderate earthquake. Under the current stress field system, the relative shear stress on the nodal plane I of the focal mechanism for the M_S4.0 Lingwu earthquake is 0.689, and on the nodal plane II is 0.817. By comparing the two nodal planes, the principal compressivestress axis and the shear stress of nodal planes, it is concluded that the shear stress on nodal plane II is relatively large due to the fact that the angle between the principal compressive stress axis and nodal plane II is larger than the angle between the principal compressive stress axis and nodal plane I. Theoretically, both the two nodal planes may be seismogenic rupture plane. However, considering the relative shear stress and right-lateral strike-slip properties of existing geological faults, this paper considers that the possibility of shear slip on nodal plane II is greater than that on nodal plane I, so it is relatively more likely that nodal plane II is seismogenic rupture plane. The principal compressive stress of regional tectonic stress is not equal to the P-axis of focal mechanism. The strike of P-axis is an inherent attribute of focal mechanism nodal plane, which is mainly related to the strike of focal plane, just like the two nodal planes of focal mechanism. The angle between the direction of the principal compressive stress and the seismic fracture plane is one of the main reasons affecting the shear stress on the fracture plane, but does not affect the strike of the P-axis.