[Objectives] This study focuses on the mechanism of action of Microbially Induced Calcite Precipitation (MICP) technology in improving the strength characteristics of expansive soil. Expansive soil, characterized by high water sensitivity and significant swelling-shrinking behavior, often causes engineering hazards. Traditional modification methods have issues such as high cost and easy contamination. The MICP technology, which utilizes microorganisms to induce calcite precipitation, can fill soil pores and cement soil particles, and is expected to enhance soil strength in a green and efficient manner.[Methods] In this experiment, Sporosarcina pasteurii (CGMCC 1.3687) was selected, and the microbial solidification was carried out using the mixing method. This study systematically explored the mechanism of the effect of different calcium ion concentrations on the strength characteristics of modified expansive soil, among which calcium ion concentration is the core variable. Through a series of consolidated quick shear tests, the internal relationship and action path between calcium ion concentration and strength indicators were analyzed, and the key mechanism by which it improves soil strength by regulating calcium carbonate formation was clarified. [Results] This study shows that MICP technology significantly enhances the shear strength of medium expansive soil, altering its mechanical behavior from the single strain mode of untreated soil to a dual-mode response: strain softening under low confining pressure and strain hardening under high confining pressure. Soil treated with Ca²⁺ solution exhibits higher shear strength than the untreated group. Specifically, cohesion (c) and internal friction angle (ϕ) follow a single-peak trend with increasing Ca²⁺ concentration, peaking at 1.5 mol/L (c = 42.5 kPa, ϕ = 18.4%)—representing a 269.6% increase in cohesion and 10.2% increase in internal friction angle. 1.5 mol/L is identified as the optimal Ca²⁺ concentration, as it maximizes calcium carbonate yield and cementation effect. Too low a concentration leads to insufficient Ca²⁺, while too high a concentration inhibits bacterial activity; both scenarios reduce calcium carbonate production and soil strength.[Conclusions] The research results verify that MICP technology can significantly enhance the strength of expansive soil, and the mechanism lies in the significant positive correlation between calcium carbonate content and strength indexes.