Abstract: Sanjiang Plain, as one of the most important grain production areas and soil carbon pools in the black soil region of Northeast China, has consistently been a research focus in soil science and ecology regarding the content, distribution characteristics, and variation trends of soil carbon in cultivated layer. Based on the data of 1:250 000 land quality survey and the Second National Soil Survey, this paper studies the distribution characteristics and spatiotemporal variation patterns of soil organic carbon(SOC) in Sanjiang Plain through geochemical assessment methods for land quality, soil carbon density calculation, and data processing and mapping with ArcGIS and SPSS. The results indicate that the organic matter in the surface soil(0-0.2 m) of the study area is generally abundant, with the average SOC content of 2.49% and total carbon content of 2.67%, displaying a pattern of high in central-eastern areas and low in west spatially, with the average organic carbon density of 6.06 kg/m² and the total carbon storage of 581.3 Tg in surface soil. The comparative analysis with the Second National Soil Survey reveals that 65.7% of surface soil organic matter shows decreasing trends, with the SOC storage decreasing by 523.1 Tg, accompanied by carbon density reduction of 5.84 kg/m². Despite the decline in average content of soil organic matter over the past 40 years, Sanjiang Plain maintains higher SOC levels than the average of Northeast China Plain, remaining one of the areas with the richest soil carbon storage in the black soil region of Northeast China.