Soil organic carbon (SOC) density and storage in the agricultural topsoil of the Songnen Plain were evaluated by the soil-type method by using data from the Second National Soil Survey conducted in 1979-1985 and field observations in 2015. The spatial distribution of SOC density was analyzed by ArcGIS, and a fitting analysis of the changes in SOC storage trend in the districts and counties of the Songnen Plain over the past 35 years was conducted using the saturation value method, after which the carbon sequestration potential of the agricultural soil was estimated. The main results are as follows: (1) the average value of the agricultural topsoil organic carbon density was 1.61 kg/m² in 2015. About 81.59% of the SOC density of the agricultural topsoil area of the Songnen Plain showed a decreasing tendency over the last 35 years, and the SOC was mainly distributed in the north, east, and southeast of the Songnen Plain; the most significant decreases were in eastern Fuyu County, central Yi'an County, western Zhaodong County, and western Fuyu County; (2) the total storage of organic carbon in the agricultural topsoil was 233.63 Tg in 2015, which decreased by about 32.62 Tg compared with that of the second national soil survey; and (3) the organic carbon sequestration potential of agricultural topsoil in the Songnen Plain was about-32.7 TgC, indicating a carbon resource area, and its average value per unit area was-1.793 × 10^-3 Tg C/km².