This study examined the influence of environmental factors on soil organic carbon density (SOCD) in five regions of southwest China, including karst canyon, karst peak cluster depression, karst plateau, karst trough valley, and non-karst areas. Significant differences in SOCD were found across these landscapes. The study quantitatively analyzed the vertical distribution and key drivers of SOCD in these landscapes. Specifically, soil organic carbon density value in soil thickness of 0 to 0.40 m is: karst trough valley>karst peak cluster depression>karst plateau>karst canyon>non-karst regions; soil organic carbon density value in soil thickness of 0.40 to 1.00 m is: karst plateau>karst trough valley>karst peak cluster depression>karst canyon>on-karst regions. Soil organic carbon density in non-karst regions is markedly positively related to slope gradient, slope aspect, and slope position and negatively correlated with land use and soil thickness. The study also found that the main factors influencing SOCD differ by region. SOCD was influenced by soil thickness in karst canyons, karst peak cluster depressions, and karst plateaus. SOCD was influenced by slope aspects in karst trough valleys.