Abstract: Insufficient selenium intake in China has received widespread attention. Nationwide land quality geochemical surveys have shown that selenium contents are relatively high in the mountainous areas of southwestern China. However, the sources of selenium and the factors influencing its enrichment in these areas remain unclear. In this study, Muchuan County, which has favorable selenium-rich conditions in the Wumeng mountainous area, was selected as the study area. A random forest model and SHAP method were used to investigate soil selenium content and its influencing factors by integrating environmental variables, including elevation (EL), annual mean land surface temperature (LST), precipitation (Prec), parent material (PMT), and soil physicochemical properties. The results showed that the mean selenium content in the soils of Muchuan County was 0.38×10⁻⁶, which was 1.8 times the background value of soils in China, indicating an overall relatively high level. The area of land with relatively abundant selenium and above was 703.59 km², accounting for 49.99% of the total area, indicating good potential for selenium-rich land resources. The random forest model showed good predictive performance, with R² values of 0.84 and 0.76 for the training and test sets, respectively. SHAP analysis showed that SOM was the dominant factor controlling the variation in Se content. Areas with high EL, SOM, and Mn were favorable for Se enrichment, whereas high Prec, LST, and pH promoted Se leaching.The research suggests that the Se enrichment in the surface soil of Muxian County is jointly controlled by the parent material of the soil, topography, climate and soil physical and chemical properties. Among them, SOM has the most significant effect on Se retention. The results of this study can provide certain basis for the evaluation and development and utilization of selenium-rich land resources in the Wumeng Mountainous Area.