Soil carbon pool is an important part of global surficial carbon pool,which will influence global climate change as a carbon source or sink for greenhouse gases.Taking areas covered by the multi-purpose regional geochemical survey(MPRGS)in the Changjiang drainage basin as an example,the soil carbon density and soil carbon storage and their changes have been calculated by combining MPRGS data and the second national soil survey data.And also,the factors that caused the carbon sinks and sources were analyzed.The results show that the soil organic carbon density(SOCD)significantly increased at most regions of the study area from the 1980s to 2000s,especially in the Longmen Mountain region of western Chengdu,southern region of Ya'an and the surrounding region of Guiyang.The amount of soil carbon increase is about 312.38 TgC. Recovery of vegetation in woodland and grassland,and improved land management in paddy fields and dry farmland are the major factors that increased soil carbon storage in the respective land use area.The land use change has relatively little impact to soil carbon stock changes in the study area.Soil erosion had almost no effect on paddy field,but it had obviously restrained the increase of SOCD in the dry farmlands.And also,the climate change had no obvious influence on soil carbon stock in this region.