Abstract: The amount of carbon fixation and its spatial and temporal characteristics by farmland ecosystem are estimated and analyzed ．The results show that the total quantities of carbo n fixation increases gradually in the past two decades be-cause the carbon density per hectare increases．The East China belongs tO the area of the high value(over 3t／hm² )and the agropastural ecotone in North China has the low value of carbon density which is less than 1t／hm² ．Thus，the cropresidue management is the key issue for enhancing carbo n sequestration．