Soil organic carbon (SOC) is one of the most significant carbon reservoirs in terrestrial ecosystems. In subtropical monsoon climate regions, changes in SOC exert considerable influence on the carbon cycle. Taking Jiangxi Province as the study area and considering soil formation factors and data availability, 37 characteristic variables were selected to develop a spatial extrapolation model of 0-100 cm soil organic carbon density (SOCD). This model enabled the mapping of SOCD distribution in Jiangxi Province for the years 1987 and 2011. This study further analyzed the characteristics of changes in total soil organic carbon (TSOC) and the carbon sequestration rate. The following conclusions were drawn: (1) Based on the statistical analysis of soil sampling results, the mean SOCD increased from 3.70 kg C /m2 to 12.52 kg C /m2 between 1987 and 2011. Specifically, the mean SOCD for forest rose from 4.37 kg C /m2 to 13.99 kg C /m2, while that of cropland increased from 2.92 kg C /m2 to 5.94 kg C /m2; (2) The random forest (RF) model demonstrated strong performance in simulating the spatial distribution of SOCD for the year 2011 (R^2 = 0.73, RMSE = 6.22 kg C /m2); however, due to limitations in available characteristic data, the simulation accuracy for 1987 was lower; (3) According to simulation results, the soil in Jiangxi Province exhibited significant carbon sink potential during the period from 1987 to 2011. The average SOCD increased from 3.62 kg C /m2 to 11.57 kg C /m2, with forest SOCD rising from 4.13 kg C /m2 to 14.01 kg C /m2; cropland SOCD increased from 2.89 kg C /m2 to 7.43 kg C /m2, and grassland SOCD rose from 2.98 kg C /m2 to 8.83 kg C /m2. Furthermore, TSOC increased from 0.605 Pg C to 1.932 Pg C, with the carbon sequestration rate ranked as: forest > grassland > farmland.