The effect of land-use changes on soil carbon stocks is an increasing concern in the context of global climate warming. In this study, we initially delineated the deforestation area of a Picea schrenkiana forest cultivated to cropland(PSC) at the north slope of the Tianshan Mountains. We then estimated the corresponding soil organic carbon (SOC) loss during the land-use change. The PSC was estimated by the potential, actual distributions of forest and cropland. The SOC content of the forest and cropland soils was obtained through field sampling and laboratory analysis. We found that the area of the PSC was 2.68 * 10~6hm~2,and the SOC loss (per unit area) caused by the cultivation of forest to cropland was 171.7 t/hm~, correspondingly. The total SOC loss in the study area was 459.70 Tg. This result implies that continuing the afforestation and reforestation programs currently implemented in the study area would increase the SOC. In addition, we found that the potential SOC sequestration is expected to increase to a greater extent in the upper soil layers than the lower layers.