To illustrate the distribution pattern of soil organic carbon density (SOCD) in a small karst watershed and its main influencing factors, this research quantitatively analyzed the spatial heterogeneity and distribution characteristics of SOCD using 2,755 thoroughly investigated soil profile samples; field point sampling, laboratory determination, and geo-statistical analysis were used, and the major influencing factors of SOCD were analyzed using a principal components analysis. The results indicated that the SOCD decreased gradually with increasing soil depth in the small karst watershed; in particular, the average SOCD was equal to 12.11 kg·m^-2 at a depth of 100 cm, which is higher than the national level. An optimal fitting model for the SOCD in this Basin was a Gaussian model, which showed a moderately strong spatial correlation. A kriging interpolation suggested that the soil carbon density (SOCD) was higher in the eastern region but lower in the southern region, exhibiting an ascending trend from the middle to the exterior. In the small karst watershed, the SOCD at a depth of 100 cm differs between different vegetation types, different soil utilization types, and different soil types. The soil thickness, rock coverage and altitude were the principal influencing factors on the SOCD in the small karst watershed, among which soil thickness had the largest impact.