【Objective】China is a country deficient in selenium (Se), because the soils in most of its provinces and regions or about 72% of its total land are under Se stress to a varying extent. However, during the soil survey completed recently, Guangxi was found to have a large tract or continuous patches of soil rich in Se, or so far the largest tract in the country. In that region, the soils in Karst areas were apparently higher in Se content than those in non-Karst areas. It is hypothesized that the tract of Se-rich soils benefit jointly from their soil forming parent rock, soil composition and unique Se enrichment mechanism. In order to validate this hypothesis, a comprehensive study was conducted. In this study, a total of 15480 topsoil samples, 452 subsoil samples and 200 rock samples were collected in Wuming County, South Guangxi, where exist large tracts of karst landform for analysis of soil physico-chemical properties, and contents of calcium oxide (CaO), potassium oxide (K₂O), sodium oxide (Na₂O), magnesium oxide (MgO), silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), Total iron (TFe₂O₃), selenium (Se), manganese (Mn), titanium (Ti), soil organic carbon (SOC) and pH.【Method】Spatial contrast, scattergraph and multivariate statistic methods were adopted to analyze sources of Se containing materials in the soil and impacts of soil forming parent rock, weathering processes, soil composition, pH and soil organic carbon (SOC) on soil Se, collate the affecting factors in sequence of priority. And furthermore, discussions were held on main causes of the formation of the Se highly-enriched soils in the key Karst areas of Guangxi. 【Result】Results show that the concentration of Se in the topsoil of the study area varies in the range of 0.07～9.04 mg•kg^-1 and its background value is 0.87 mg•kg^-1, which is 4.36 times that of the country. Soil Se demonstrates a good spatial coupled relationship with the stratum and is strongly and positive related to soil Se in the subsoil layer, which suggests that soil Se in the topsoil comes from underlying soil layers; scattergraph indicates that soil Se was affected by weathering intensity (CIA), Al₂O₃, TFe₂O₃, SiO₂, SOC, pH, CaO, K₂O, Na₂O and MgO. Multivariate fitting with linear regression equation and equation error testing reveal that CIA, clay minerals (Al₂O₃), and Fe-bearing minerals played leading roles in secondary enrichment of Se, while SOC, K-bearing minerals, Na-bearing minerals, Mg-bearing minerals followed in the function, and pH, Ca-bearing and Mn-bearing minerals did in the last with very weak effects. 【Conclusion】Based on the findings about sources of Se and relationships between Se enrichment affecting factors, it is concluded that weathering processes dominates Se enrichment in the soils in Karst areas, while the high contents of clay minerals and Fe-bearing minerals in the soil play leading roles in sorption of Se, which may be the main reason why high Se enrichment occurs in the Karst area of Guangxi where limestone is lower in Se content than the average rock in China.