To research soil organic carbon (SOC) in a typical small karst basin of western Guizhou in southwest China, data from the second national soil resource survey (1980) and data analysed in the laboratory in 2015 were used. This paper examines the changes in soil organic carbon density (SOCD) and soil organic carbon stock (SOCS) in the topsoil (0-20 cm) over the past 35 years based on soil types, and the primary influencing factors are also discussed. The SOCD and SOCS slightly increased over this period. The SOCD increased from 4.91 kg m(-2) to 5.13 kg m(-2), and the SOCS increased from 368.27 x 10(3) t to 385.09 x 10(3) t. The basin sequestered a low level of carbon during this time. Paddy fields were the key contributor to the increases, and the SOCD and SOCS of paddy fields increased by 1.61 kg m(-2) and 32.39 x 10(3) t, respectively. Generally, the SOCD and SOCS in the soils from the southern part of Houzhai Basin increased considerably, and those from the northern part of the basin decreased significantly. The spatial variation of SOCD in the Houzhai Basin was mainly due to natural factors. However, the temporal change of SOC was primary caused by human activities.