[Objective] The effect of afforestation on soil organic carbon density (SOCD) was studied to provide a scientific basis for improving the carbon sink of terrestrial ecosystems in the forest-steppe ecotone of Northern Hebei Province. [Methods] Scotch pines (Pinus Sylvestris var. mongolica) and larch (Larix principis-rupprechtii) plantations were selected as the research subjects in the Yudaokou area located in a forest-steppe ecotone. Unforested lands (including unforested land Ⅰ with high biomass, unforested land Ⅱ with medium biomass, and unforested land Ⅲ with low biomass) served as controls. The soil organic carbon (SOC ) content and soil organic carbon density (SOCD) of different terrestrial ecosystems were studied. [Results] ① The SOC content of unforested lands in Yudaokou area ranging from 2.54 to 60.57 g/kg were positively correlated with the vegetation biomass. ② At the same age (16—20 years), the SOC content in the L. principis-rupprechtii and P. Sylvestris var. mongolica plantations was significantly higher than that in unforested land Ⅱ and unforested land Ⅲ and lower than that in unforested land Ⅰ (p<0.05), and the SOC content in the L. principis-rupprechtii plantations was higher than that in the P. Sylvestris var. mongolica plantations, with a significant difference observed in the 0—10 cm soil layer (p<0.05). ③ The order of SOCD (0—60 cm) from high to low was unforested land Ⅰ (127.56 t/hm²)＞L. principis-rupprechtii plantation(105.26 t/hm²)＞P. Sylvestris var. mongolica plantation (75.52 t/hm²)＞ unforested land Ⅱ (56.06 t/hm²)＞ unforested land Ⅲ (31.78 t/hm²), and there were significant differences between the vegetation types (p＜0.05) except that between P. Sylvestris var. mongolica plantations and unforested land Ⅱ (p>0.05). ④ The SOCD of 0—60 cm in the L. principis-rupprechtii plantations increased with the increase of the ages in all soil layer, and they were 105.26 t/hm² (16~20 a), 112.29 t/hm²(21~25 a) and 159.73 t/hm² (26~30 a), respectively; the SOCD of the P. Sylvestris var. mongolica plantations decreased first and then increased with the increase of the ages, and the SOCD of 0—60 cm soil depth was 79.38 t/hm²(6～10 a), 54.24 t/hm²(11～15 a), 75.52 t/hm²(16～20 a) and 82.24 t/hm²(＞20 a), respectively. [Conclusion] The effect of afforestation on SOC content and storage depends on the initial conditions of forested land and the tree species; in Yudaokou area, afforestation using L. principis-rupprechtii and P. Sylvestris var. mongolica pine in places with lower SOC content can increase SOCD, and L. principis-rupprechtii can increase SOCD more than P. Sylvestris var. mongolica.