This study investigated the spatial distribution characteristics of selenium in surface soils and its primary influencing factors in northeastern Gangcha County. Based on selenium content data from 2,423 surface soil samples, it comprehensively employed descriptive statistics, geostatistics, Kruskal-Wallis tests, and correlation analysis. Results indicate: Topsoil selenium concentrations ranged from 0.098 to 1.790 mg/kg, with an average of 0.243 mg/kg ± 0.124 mg/kg; Based on content classification standards, most areas exhibited “adequate” selenium levels, with scattered high-value zones distributed patchily throughout. Localized areas in the central-northern region showed “deficient” selenium levels. Significant variations in selenium content were observed among soils derived from different parent materials, with those developed from fluvial-alluvial deposits exhibiting higher selenium content, averaging 0.281 mg/kg ± 0.167 mg/kg. By soil type, surface soil selenium content decreased in the following order: Chestnut Calc-Sole > Meadow Soil > Alpine Meadow Soil > Meadow Bog Soil > Alpine Shrub Meadow Soil. Selenium content increased significantly with rising pH, with strongly alkaline soils (pH ≥ 8.5) exhibiting markedly higher selenium levels than other types. A statistically significant positive correlation existed between organic matter content and selenium concentration (ρ = 0.31023, p < 0.0001).