Heavy metal contamination poses significant risks to ecosystems and human health. This study comprehensively investigated the mechanisms involved in the simultaneous removal of lead (Pb) and chromium (Cr) composite contaminants using microbially induced carbonate precipitation (MICP) technology. The optimal growth conditions for Sporosarcina pasteurii were determined as follows: a urea concentration of 0.5 mol/L, a Ca²⁺ concentration of 20 mmol/L, pH of 8, temperature of 30℃, and a carbon source concentration of 10 g/L. Under these specified conditions, MICP achieved synergistic removal efficiencies exceeding 98.98% for Pb(Ⅱ) and 82.48% for Cr(Ⅵ) in the composite contamination system. Analyses utilizing X-ray diffraction (XRD), scanning electron microscopy—energy dispersive spectroscopy (SEM-EDS), and Fourier transform infrared spectroscopy (FTIR) confirmed that Pb(Ⅱ) was primarily immobilized through carbonate precipitation and mineralization, whereas Cr(Ⅵ) followed a dual pathway involving biological reduction and carbonate co-precipitation. Specifically, Cr(Ⅵ) was initially reduced to less toxic Cr(Ⅲ) on the bacterial surface, which subsequently reacted with carbonate ions to form insoluble (Cr,Ca)CO₃ compounds. This study provides a sustainable biomineralization strategy for the remediation of heavy metal composite pollutants.