Abstract: To enhance the utilization of industrial solid wastes and improve the stabilization efficacy of expansive soils, this study employs fly ash as the precursor, with phosphogypsum and sodium hydroxide (NaOH) as activators. The Box-Behnken Design (BBD) of response surface methodology (RSM) is adopted to optimize the proportions of fly ash, phosphogypsum, and NaOH. The unconfined compressive strength (UCS), X-ray diffraction (XRD), and scanning electron microscopy (SEM) are used to systematically investigate the stabilization mechanisms and microstructural evolution of expansive soil. Experimental results reveal that the optimal mixing ratio is 18% fly ash, 8% phosphogypsum, and 2.8% NaOH. The strength of stabilized soil reaches 0.65 MPa at 7 days and 0.79 MPa at 28 days. RSM optimization further refines the ratio to 17.978% fly ash, 7.036% phosphogypsum, and 3.043% NaOH, yielding UCS values of 0.67 MPa (7 d) and 0.83 MPa (28 d). The minimal deviation between predicted and experimental results validate the accuracy and reliability of RSM for mixture design. Microscopic analysis identifies AFt, C-S-H, and C-A-S-H as the primary hydration products at both curing ages. The hydration products effectively fill and encapsulate soil pores, improve soil compactness and strengthen the skeletal structure, which reduces the expansibility of the stabilized soil and improves its overall strength. This research establishes a theoretical and experimental foundation for alkali-activated fly ash-phosphogypsum stabilization of expansive soils.