97 soil profiles were surveyed across the Qilian Mountains using a purposively sampling strategy. Vertical variations in profiles of soil organic carbon (SOC) and their correlation with various environmental variables (mean annual precipitation and temperature, altitude, slope gradient and aspect, NDVI) and soil attributes (particle size composition and bulk density) were analyzed. Results showed that there were 4 kinds of SOC profile distribution modes: homogeneous, surface- accumulated, gradually-decreasing and chaotic modes. Both SOC content and density had moderate to strong spatial variability, and the variation coefficient increased with depth. Analysis of random forest proved that the surface soil layer (0-20 cm) had the best model performance with R² reaching 0.68 and 0.52 for SOC content and SOC density respectively, and R² decreased with increasing soil depth. Moreover, the environmental factors of NDVI and mean annual precipitation were the main influential factors for variations of surface SOC content and density, but the influence of soil attributes on SOC content and density was intensified with increasing soil depth.