Taking the oasis of lower Sangong River watershed in the Tianshan Mountain northern slope as study area, six kinds of cropping systems, including those of saline-alkaline tolerance crop, food crop, melon and vegetables, economic plants-cotton, economic plants-grape, and economic plants-hop, were selected based on the investigation of local cropping systems, and the dynamic change of soil organic matter under effects of these cropping systems was analyzed by using the laboratory data in 1982, 1999 and 2003. The results showed that land policy and market price had a direct effect on the transformation from annuals cropping system to perennials cropping system, while soil organic matter (OM) content experienced a decline in 1982-1999 owing to the transformation from planting other crops to planting cotton, and a rise in 1999-2003 resulting from the conversion from planting food crops and cotton to planting perennial crops. On the whole, the soils in the oasis presented as a carbon sink, suggesting that the change of artificial vegetation in the oasis was helpful to the accumulation of soil OM, and beneficial to the sustainable use of farmland.