Ecological threats from land aggregate mining and the underutilization of marine silty sand due to its poor mechanics necessitate sustainable improvement techniques. This study introduces a novel two-stage Microbially Induced Calcium Carbonate Precipitation (MICP) injection strategy. Stage 1 fostered the formation of CaCO₃ on the surface of the sand, while stage 2 promoted CaCO₃ bridging and pore-filling to reinforce the sand. The method's effectiveness was evaluated through relative density, direct shear, and microstructural tests. The treatment yielded significant gains, increasing shear strength from 24.67 kPa to 98.45 kPa and relative density from 12.86% to 27.77%. Microstructural analysis confirmed that CaCO₃ precipitation effectively bonded particles and filled pores, imparting rock-like strength. The findings establish staged biocementation as a promising method for transforming marine silty sand into a viable construction material for coastal infrastructure. Future research should focus on large-scale field validation to confirm its practical value.