Abstract: To address the poor engineering performance of fine-grained silty soil, the biobased polymer sodium alginate (SA) was used to reinforce the soil, and its microscopic solidification mechanism was investigated. The effects of different SA contents on the unconfined compressive strength (UCS) and shear strength of fine-grained silty soil were studied, and the microstructure, particle size distribution, and morphology after solidification were analyzed via XRD, particle size analysis, SEM, and FTIR. The results show that SA can significantly improve the strength of silty soil. Among them, the 28-day UCS of the sample with 1.5% SA increased from 70.35 kPa to 365.43 kPa, and the shear strength under 400 kPa normal stress increased from 188.3 kPa to 223.6 kPa. The strength improvement is due mainly to the increase in cohesion. The microscopic results indicate that SA forms a gel network structure in the soil, promotes particle aggregation, fills pores, and interacts weakly with particle surfaces through functional groups such as carboxyl and hydroxyl groups, thereby constructing flexible cementation and an organic–inorganic composite bonding system. The research results provide a theoretical basis and technical support for the application of natural polymer-modified materials in the green stabilization of silty soil.