On October 15,2021,a M_S4.7 earthquake occurred in Arhorchin Banner,Chifeng City,Inner Mongolia. It was the second moderate earthquake in this area since the M_S5.9 earthquake in Arhorchin Banner on August 16,2003. The focal mechanism solution of this earthquake was obtained by CAP method. Compared with the focal mechanism solution of the 2003 M_S5.9 earthquake,it was found that the focal mechanism solutions of the two earthquakes were basically consistent. The strikes of nodal plane II are NW,and the strikes of the two earthquakes are respectively NW. Combined with geological structure data,it is determined that Shuiquanzigou-Tianshankou fault is the seismogenic structure of two earthquakes. In our study,the reliable focal mechanism solutions of 22 M_L≥3.5 earthquakes in and around the focal area since 2012 are obtained. We find that the focal mechanism solutions in Tongliao-Arhorchin area are all strike-slip type. The inversion results of stress field show that the stress field in this area is jointly affected by the NNW tension and NEE compression. The stress shape factor R=0.7 suggests that the intermediate stress field is nearly in EW direction,with tensile properties,and the tectonic environment is mainly tensile. The M_S4.7 earthquake may be a medium-intensity earthquake caused by strike-slip dislocation of Shuiquanzigou-Tianshankou fault under the action of tension in NNW and EW direction and compression in NEE direction.