Abstract: In order to improve the poor engineering performance of low liquid limit silty soil, a bio-based polymer—sodium alginate (SA), was used as a reinforcement material. This study systematically investigated the unconfined compressive strength (UCS) and shear strength of low liquid limit silty soil with varying SA contents. Furthermore, the structure, particle size distribution, and microstructure of the SA-treated soil were analyzed using XRD, particle size analysis, SEM, and FTIR. The results indicate that the addition of SA significantly enhances the strength of the silty soil. After 28 days of curing, the UCS of samples with 1.5% SA increased from 70.35 kPa to 365.43 kPa, while the shear strength under a normal stress of 400 kPa rose from 188.3 kPa to 223.6 kPa. This strength improvement is attributed mainly to an increase in cohesion. Within the soil matrix, SA forms a gel network structure that promotes particle agglomeration and fills pores. And through weak chemical interactions involving carboxyl, hydroxyl, and other functional groups on particle surfaces, a flexible cementation system and an "organic-inorganic" composite connection are established. The findings provide a theoretical basis and technical support for the application of natural polymer-based materials in the environmentally friendly stabilization of silty soil.