This article takes Wulong Mountain in Longshan County, Hunan Province, as the research area. Based on 1∶50000 land quality geochemical survey data, a comprehensive evaluation is carried out around four aspects: soil selenium enrichment degree, environmental quality, irrigation water quality, and crop selenium content. A natural and pollution- free selenium rich land area is delineated, and the causes of soil selenium in this area are analyzed to explore the sources and influencing factors of soil selenium.Results: ① The soil Se content in the study area ranges from 0.19 μg/g to 0.84 μg/g, with an average and median value of 0.48 μg/g and 0.45 μg/g, respectively, which is 2.24 times higher than the surface soil in China. The land area with soil Se content greater than 0.4 μg/g is 15.18 km2 (22770 acres), accounting for 72% of the research area, and has high prospects for selenium rich development. ② The content of elements such as As, Hg, Cr, and Pb in the soil of the research area is lower than the screening value of GB15618- 2018 Soil Environmental Quality and Agricultural Land Soil Pollution Risk Control Standard. The proportion of Cd element content is 62% lower than the screening value. The soil environmental quality is excellent, the irrigation water quality is good, and the selenium rich rate of agricultural crops is 80%. ③ There is a significant positive correlation between Se and Cd, Cu, Hg, Ni, Pb, Zn, P, N, and organic carbon in the soil of the study area, while there is a negative correlation with pH. In the vertical direction of the soil profile, the soil Se content is mainly the result of the comprehensive effect of various factors such as organic carbon, clay minerals, and pH, exhibiting surface aggregation characteristics. Conclusions: In this study, a total of 13251 acres (1 acre=2000/3 m2) of natural and pollution- free selenium rich land were designated. Through analysis of the causes of Se, it was found that the Se element in the natural selenium rich soil in the study area comes from the alluvial soil parent material of black carbonaceous siliceous rock layers, and the source is stable. The results of this study can provide a theoretical basis for the cultivation of local selenium rich agricultural products, the development of selenium rich industries, and support for rural revitalization.