In this paper, the expansive soil in the experimental section of the water diversion project from the Yangtze River to the Huaihe River has been selected as the research object, and a strain of microorganism has been selected from the non-expansive soil to turn the strong expansive soil into weak-medium expansive soil. The colony morphology, Gram staining morphology and 16S rRNA sequencing are observed, and the microbial strain is identified as Bacillus subtilis. With various experimental conditions of cultivating Bacillus subtilis, the orthogonal experimental design has been carried out to obtain the optimal culturing conditions of Bacillus subtilis: 40% water content, 3% bacteria incorporation, 5℃ curing temperature and initial pH7.5. Under this condition, the maximum reduction of free expansion rate is 21.7%.