【Objective】This study aimed to analyze the response of soil carbon composition to long-term nitrogen addition in the Stipa baicalensis meadow steppe. 【Method】A nitrogen addition experiment was designed in the meadow steppe of Ewenke Banner, Hulunbuir City, Inner Mongolia in 2010, and 8 nitrogen treatments were set as 0 (N0), 15(N15), 30(N30), 50(N50), 100(N100), 150(N150), 200(N200) and 300(N300) kg·hm-2·a-1(in N). Soil samples were collected in August 2019 and soil organic carbon and its fractions were measured to investigate the changes and drivers of soil organic carbon and its fractions after 10 years of nitrogen addition. 【Results】The results showed that: (1) Compared to the control, soil organic carbon (SOC) content and soil refractory components (RP-C) did not change significantly under long-term nitrogen addition, but activated carbon component I (LPⅠ-C) and II (LPⅡ-C) increased by 0.48%-15.59% and 1.94%-8.41%, respectively. Soil easily oxidized organic carbon (EOC) did not respond, whereas the contents of dissolved organic carbon (DOC) and microbial biomass carbon (MBC) changed significantly. The overall soil carbon composition was more sensitive to N addition at the level of 30, 50 and 100 kg·hm-2 ·a-1; (2) The responses of sensitivity index (SI) of soil organic carbon to nitrogen addition showed that MBC was more sensitive to long-term nitrogen addition, which could be used as an indicator suggesting the changes in organic carbon component in the region; (3) Structural equation modeling (SEM) indicated that soil carbon components were regulated mainly by plant biomass and soil pH under long-term nitrogen conditions. 【Conclusion】Nitrogen addition increases soil active carbon content in Stipa baicalensis and the changes in soil organic carbon and its components are mediated mainly by soil pH and plant biomass.