This paper studied the size and the dynamics of different land use soil organic carbon in Xiashu and discussed the influence on soil carbon pool of land use change,based on the three pool theory.The results indicate: The total soil organic carbon for 0~20 cm,20~40 cm and 70~100 cm layers are 7 818.34~9 242.30 mg/kg,3 093.66~3 338.59 mg/kg and 2 337.27~3 188.40 mg/kg.The slow carbon pool in all layers reduced when the land use changes from forest to dry field; and the passive carbon pool increased in mid-layer.These differences will augment with the increase of cultivation time.The proportion of each carbon pool has no obvious change.The proportion for active,slow and passive pool are 0.41% ~3.81%,23.23%~79.11%,18.11%~74.80%.The mean residue time for active and slow pool are 3~40 days and 5~24 years.Soil organic carbon decomposition has two phases: fast decomposition in early time and slow decomposition in late time.The fast decomposition phase is the mean residue time of active carbon.The authors have the point of view that the dry field is more powerful than forest in carbon sequestration.The dry field will be a large carbon sink under good management and effective tillage control measure.