In this paper, Caragana erinacea shrub grassland in western Sichuan was taken as the research object. The changes of soil physical and chemical properties and soil organic carbon (SOC) mineralization in 0-10 cm, 10-20 cm and 20-40 cm under different expansion degrees of Caragana erinacea shrub (native grassland, moderate shrub expansion, and heavy shrub expansion) were analyzed, and the influence mechanism of shrub expansion on SOC mineralization in alpine grassland was explored. The results showed that: (1) Shrub expansion had no significant effect on soil total nitrogen (TN) content, significantly increased SOC content in each soil layer, significantly increased mineral mineral-associated organic carbon (MAOC) content and its proportion in SOC in 10-20 cm soil, and significantly decreased soil water content (SWC) in 0-20 cm soil. In the stage of heavy shrub expansion, the content of microbial biomass carbon (MBC) in 10-40 cm soil layer was significantly increased, and MBC/SOC and MAOC/SOC in 0-10 cm soil layer were significantly decreased. (2) Shrub expansion significantly reduced the potential mineralized carbon pool (C₀) of soil organic carbon in 0-10 cm soil layer, and the C₀ in 10-40 cm soil layer was significantly increased in the expansion stage of heavy shrubs. (3) Correlation analysis showed that SOC mineralization process was significantly positively correlated with SWC, SOC, TN, MBC and MAOC, and significantly negatively correlated with MAOC/SOC. Stepwise regression analysis showed that the SOC mineralization had the strongest correlation with MBC and SWC, which could effectively explain 91.1% of the change of C₀. It can be inferred that the change of SOC mineralization in this area after Caragana erinacea shrub expansion is mainly due to the influence of shrub expansion on MBC and SWC. The decrease of soil superficial water affects microbial activity and weakens SOC mineralization, while the increase of microbial biomass in shrub root development layer strengthens SOC mineralization.