Abstract: Using spatio-temporal substitution method, this study analyzed soil organic carbon (SOC), soil total nitrogen (STN), soil total phosphor (STP), root biomass and litter biomass of Caragana microphylla plantations of different ages. Soil carbon content change characteristics under C. microphylla plantations and the influencing factors in the Loess Hilly Semiarid Region were discussed. It threw further light on soil carbon mechanisms under C. microphylla plantations. The results suggested that at 0~60 cm soil profile, SOC content steadily decreased with soil depth increasing. Also SOC content in the 0~20 cm soil layer was higher than that in the other soil layers. At 0~20 cm soil layer, SOC content was 2.68~11.44 g·kg^-1. At 40~60 cm soil depth, however, SOC content fluctuated only minimally and was 1.64~2.73 g·kg^-1. Compared with two-year abandoned sloping farmland (CK), variations in average SOC content under C. microphylla at 0~60 cm soil depth initially decreased and then increased before finally stabilizing with time. At C. microphylla ages 10 and 17, SOC content significantly decreased by 34.5% and 26.9%, respectively, compared with that in CK. At C. microphylla age 26, SOC content significantly increased, which was 1.43 times as that in CK. At C. microphylla ages 40 and 50, however, SOC content was in the state that accumulation and consumption of SOC was relatively stable. Correlation analysis showed that SOC was significantly correlated with STN, root biomass and litter biomass. However, there was no correlation between SOC and STP contents. The results indicated that soil carbon sequestration capacity increased by increasing soil total nitrogen content. Both the amounts of root biomass and litter biomass dictated soil carbon sequestration degree.