Abstract: The content of soil organic carbon (SOC) in forest lands are different between regions and soil depths. However, the effect of tree growth on the change of SOC along the precipitation gradient was still unclear. Based on the matured forest of Pinus sylvestris var. mongolica in different precipitation areas of Liaoning Province, we analyzed the difference of SOC and their ratio (SOCR) in different soil depths and areas between forest land and control land. The results showed that SOC and SOCR were significantly affected by area, soil depth and their interactions. The content of SOC in the forest land was significantly greater than that in the control land, and the content of SOC in the humid and semi-humid areas was greater than that in the semi-arid areas. The content of SOC was decreased with the increase of soil depths. The SOCR in the humid area was significantly higher than that in the semi-humid and semi-arid area. Therefore, the cultivation of Pinus sylvestris in the plantation could significantly improve SOC, especially in the surface soil.