Abstract: A composite stabilization system using a slag-alkali activator is developed for marine soft soil characterized by high water content and low strength. Unconfined compressive strength (UCS) tests, scanning electron microscopy (SEM), and X-ray diffraction (XRD) analysis are conducted to investigate the effects of stabilizer dosage and curing age on the mechanical properties of the treated soil and to reveal the solidification mechanism. Results indicate that the unconfined compressive strength increases with increasing stabilizer content. The strength exhibits favorable long-term stability with curing age. This improvement is attributed to the alkali activator, which reacts with slag to form high-strength cementitious products that densely cement the soil particles, thereby enhancing overall strength and stability. Combining technical advancement with environmental friendliness, the proposed scheme provides a scientific basis and technical support for material selection, mix design, and construction optimization in soft soil stabilization projects.