[Objective] The effectiveness of microbially induced carbonate precipitation (MICP) in saline soils is frequently compromised by the high-salt environment. The presence of salt ions inhibits microbial activity and disrupt ion migration, leading to reduced calcium carbonate precipitation efficiency and non-uniform spatial distribution. These limitations hinder the enhancement of mechanical properties in the stabilized soil and undermine the reliability of engineering applications. To address these challenges, this study introduces sodium alginate (SA) to develop an SA-MICP composite stabilization system aimed at improving the curing performance of saline soil. [Methods] The research focuses on investigating the variation patterns of calcium carbonate precipitation behavior and mechanical properties under different SA dosages, while elucidating the mechanism of SA-mediated synergistic regulation of microbial mineralization through multi-scale experimental approaches. Sodium alginate was incorporated at varying mass fractions (0% to 6%) relative to the dry soil weight. The quantity of precipitated calcium carbonate was quantified using both solution-based precipitation assays and soil column reaction tests. Mechanical performance of the stabilized soil at curing ages of 3, 7, and 14 days was evaluated via unconfined compressive strength (UCS) and Brazilian tensile strength (BTS) tests. The morphology, mineralogical composition, and spatial distribution of calcium carbonate were systematically analyzed using particle size analysis, scanning electron microscopy (SEM), and X-ray diffraction (XRD). [Results] The incorporation of sodium alginate significantly enhanced the MICP-based stabilization of saline soil, with the extent of improvement being strongly dosage-dependent. Calcium carbonate precipitation initially increased with SA dosage, reaching a maximum at 3% SA, where the yield was approximately 58% higher than that achieved by MICP alone. Beyond this threshold, further increases in SA dosage resulted in decreased precipitation, suggesting that excessive SA elevated system viscosity and excessively sequestered calcium ions, thereby impairing reaction efficiency. Mechanical test results aligned with this trend: both UCS and BTS increased with SA dosage up to 3%, peaking at 14 days of curing. Specifically, UCS improved by approximately 35.2%, while BTS exhibited a substantial increase of up to 194.8%. The pronounced enhancement in tensile strength indicated that SA played a critical role in improving fracture resistance and inter-particle bonding continuity. [Conclusion] Microstructural analyses further validate the synergistic stabilization mechanism of the SA-MICP system. SA-MICP-treated soil displays a continuous and dense “crystal bridge-encapsulation” cementation structure, with more uniform calcium carbonate distribution effectively filling interparticle voids. This study demonstrates that sodium alginate effectively synergizes with MICP to overcome key challenges such as low mineralization efficiency and heterogeneous precipitation in saline environments. The overall SA-MICP stabilization process follows a synergistic pathway characterized as “microbial mineralization-polymer-confined regulation-crystal bridging cementation,” ultimately yielding a densely structured soil matrix with superior mechanical performance. At a 3% SA dosage, the system achieves optimal balance between precipitation efficiency and mechanical enhancement, particularly excelling in tensile strength and crack resistance. However, excessive SA adversely affects reaction kinetics and structural homogeneity, underscoring the importance of precise dosage control. Overall, the SA-MICP composite approach significantly improves the applicability and engineering reliability of MICP in high-salt conditions.