In this paper,we utilize the CAP(Cut and Paste) method to determine the focal mechanisms of 259 earthquakes with magnitudes of 3.0 or greater in northwest Yunnan,spanning the period from May 2013 to 2022. We apply the damping parameter method to invert the regional stress field and analyze the seismic source mechanisms in conjunction with the Yangbi M_S6.4 earthquake cluster. The key findings are as follows: ① The focal mechanisms in northwest Yunnan are predominantly strike-slip,with some normal faulting. Earthquakes generally occur at shallow depths and have high dip angles. The P-axis orientation varies from NNW to NW and NE,with a noticeable clockwise deflection from north to south. Both the P-axis and T-axis exhibit small inclination angles,indicating that the stress at the earthquake sources is mainly horizontal. ② The stress field in northwest Yunnan is primarily strike-slip. In the northern part of the study area,near the Deqin-Zhongdian fault,tensile stress dominates,with the principal compressive stress axis oriented NW. In the northeastern region,near the Dali-Lijiang fault,both strike-slip and tensile stress fields are present,with tensile stress concentrated on the southwest side and the principal compressive stress axis oriented NNW. Near the Honghe fault in the Dali area,the principal compressive stress axis shifts from NNW to NNE. In the southwestern region,particularly in the Ruili-Baoshan and Tengchong areas,the stress field is strike-slip,with the principal compressive stress axis oriented NE. Overall,the principal compressive stress axis rotates clockwise from north to south,with a marked shift from NNW to NNE near the Honghe fault in the Dali area. ③ The seismic tectonics of several magnitude 5 or greater earthquakes near the Vexi-Qiaohou fault share common characteristics. These earthquakes are associated with hidden or secondary faults west of the Vexi-Qiaohou fault. The observed changes in seismicity mechanisms in 2022 may be related to the complex stress adjustments near the fault. This area warrants further study and monitoring for potential moderate to strong seismic activity. These findings contribute to a deeper understanding of the complex seismic dynamics in northwest Yunnan and highlight the importance of continued monitoring and research in the region.