【Objective】The aim of analyzing the spatial distribution characteristics and driving factors of soil organic carbon density on a large spatial scale in the water source area of the Middle Route of China South-to-North Water Diversion Project is to reveal its spatial differences and the changes with the altitude, soil types, land use, and forest types. This work should provide a scientific basis for the rational utilization of land resources and soil organic carbon (SOC) management in the area. The spatial distribution and driving factors of soil organic carbon density (SOCD) were explored to provide a scientific basis for the accurate assessment of the ecosystem carbon cycle in the climate transition zone.【Method】Based on a comprehensive assessment of spatial factors such as the elevation, soil types, and land use, the spatial variations of SOCD in this region were evaluated by the field sampling and indoor analysis with the help of a geographic information system (GIS). In addition, the main driving factors affecting the spatial distribution of SOCD in the study area were determined by analyzing the explanatory power of various influencing factors on soil organic carbon density and the difference in the degree of interaction among various factors using the geographic detector model. 【Result】(1)The results showed that the average SOCD in 0-20 cm and ≥20-40 cm soil layers was 4.18 kg/m² and 2.67 kg/m² respectively, in which the SOCD of 0-20 cm soil layer was 56.55% higher than the national average level (2.67 kg/m²).(2)Over the entire region, the SOCD was higher in forests in the northern and southern areas and lower in farmlands and grasslands in the middle area. The soil organic carbon density greater than 10 kg/m² is concentrated in the forest land with an altitude of ≥1 500-2 000 m, and less than 1 kg/m² is concentrated in grasslands with altitudes <500 m. The SOCD first increased and then decreased with an increasing elevation. The SOCD of 0-20 cm and ≥20-40 cm soil layers showed peak values (7.32 kg/m² and 4.94 kg/m²) at an altitude of ≥1 500-2 000 m. In terms of soil types, the average SOCD of limestone soil is the greatest and that of cinnamon soils is the smallest. The SOCD storage of yellow-brown earths and brown earths in the 0-20 cm soil layer was the highest, 2.005 Pg and 0.815 Pg, respectively, accounting for 72.83% of the total storage. In terms of land-use types, the forest and farmland were the main land-use types. The SOCD of 0-20 cm and ≥20-40 cm soil layers in the forest land were 4.87 kg/m² and 3.05 kg/m², respectively, and that of farmlands were 2.75 kg/m² and 2.00 kg/m², respectively. These levels were 77.09% and 52.50% lower than that of the forest land, and the SOC storage of the forest land accounted for 87.48% of the total C storage, followed by farmlands (12.02%).(3)The most powerful explanation for the spatial distribution of SOCD was the altitude (0.25) and land use (0.20), followed by soil clay (0.11). The explanatory power of the different driving factors under interaction was significantly higher than that of a single factor. 【Conclusion】The altitude and land use were the main driving factors affecting the spatial patterns of SOCD in this region. After the interaction of different driving factors, there was a synergistic effect of the two factors. The SOCD of farmlands was significantly lower than that of the forest land. Therefore, the ecological engineering construction in the study area should be strengthened, the forest protection and vegetation restoration should be carried out, and the soil carbon storage capacity of the water source in the Middle Route of China South-to-North Water Diversion Project should be improved.