Based on the analyzing of the carbon cycle under different ecosystem in Liupan mountain forest zone and the conception of carbon sink and source , the impacts of land use change on the carbon sink of ecosystem were assessed. It is found that the function of ecosystem carbon source or sink under different land uses includes carbon storage, input and output of ecosystem carbon pool, net change of carbon storage and stabilization of soil organic carbon (SOC). The ecosystem carbon storage under natural secondary forest or plantation ecosystem is higher than that under cropland or rangeland ecosystem . The storage of ecosystem carbon, SOC, active SOC pool, slow SOC, protected and unprotected fraction of SOC and labile fraction of SOC under secondary forest or plantation ecosystem are higher than those under cropland and rangeland ecosystem. Natural secondary forest ecosystem is strong carbon sink, while plantation ecosystem is weak carbon sink, and cropland or rangeland ecosystem is source. The measures of increasing ecosystem sink include increasing input of ecosystem carbon pool, decreasing output of ecosystem carbon pool and increasing stabilization of SOC in ecosystem.