It is important for the study on soil carbon sequestration to figure out soil carbon pools and their significant influencing factors in different periods. Based on the data of the second National Soil Survey of Bei’an and Kedong area in the early 1980’s, the data of the soil testing for fertilizer recommendation in 2010, and the data of the analysis of actual supplementary soil samples, soil organic carbon pool and spatio-temporal variation of its density was studied using the soil type-GIS correlation method, meanwhile, impact factors of the spatial variation of soil organic carbon density and variation of their significances in the different periods. Results show that the soil organic carbon density and its decreasing rate in the farmlands in valleys and low-lying plains is higher than that in rolling hill high plains in the study region and the carbon pool in the surface soil layer and the profile decreased at an annual average rate of -0.14 t hm^-2a^-1 and -0.13 t hm^-2a^-1, respectively. The soil carbon stock showed an apparent declining trend. In 1980, soil type (subtype), elevation, pH, and total phosphorus were found to be the main impact factor of the soil organic carbon density in the surface soil layer of the farmlands, while in 2010, soil type (subtype) and soil pH remained, but slope degree replaced elevation and total P as major impact factors.