Taking the mild burned area of Great Xing’an Mountains as research objects, the impact of burning on the soil organic carbon fractions in Pinus sylvestris var. mongolica natural forest was investigated through a comparative study. The results showed that the mild burning changed the composition and content of the soil organic carbon of Pinus sylvestris var. mongolica natural forest. After mild burning, the content of soil organic carbon, readily oxidizable carbon and particulate organic carbon decreased by 8.52 g/kg, 1.36 g/kg and 5.85 g/kg respectively at 0—5 cm soil layer, which decreased by 4.78 g/kg, 0.19 g/kg and 2.98 g/kg respectively at 5—10 cm soil layer. The difference reached a significant level between burned and unburned natural forest （P＜0.05）. The black carbon content at soil surface significantly increased after mild burning. The black carbon content of burned area at 0—5 cm soil layer increased by 9.95 g/kg, and the difference reached a significant level with unburned natural forest （P＜0.05）. In 0—5 cm and 5—10 cm soil layer, the proportion of black carbon to soil organic carbon in burned area increased by 25.4% and 6.12% respectively, the proportion of readily oxidizable organic carbon to soil organic carbon decreased by 1.49% and 0.65% respectively, the difference between burned area and unburned natural forest reached a significant level（P＜0.05）. Mild burning had little effect on the proportion of particulate organic carbon to soil organic carbon. Regression analysis showed that the soil organic carbon displayed an extremely significant linear correlation with soil organic carbon fractions both mild burned and unburned natural forest（P＜0.01）.