Abstract: Due to its strong hydrophilicity, the expansive soil will show significant volume expansion when soaked in water, which can cause hazard to the structures such as channels and embankments. Based on the capability of microbial induced calcite precipitation (MICP) to stabilize and improve the structure of soil body, the experimental and mechanism researches are conducted on the expansive soil treatment by Sporosarcina pasteurii through MICP. The results show that treating the expansive soil with Sporosarcina pasteurii can effectively suppress the swelling deformation of the expansive soil. The swelling inhibition can be up to 99.8% under the optimized microbial solution dosage. The free ions such as sodium, calcium, manganese and aluminum from the expansive soil mineral will form calcite particles through the reactions with the bacteria solution which can cement the soil particles, while the bacteria activities will induce the transformation of montmorillonite in the expansive soil into illite, which significantly inhibits the swelling capability and volume deformation of the expansive soil.