In order to study the effect of alkali-excited low-calcium fly ash on the improvement of weak expansive soil, the effects of different amounts of alkaline excitation agent NaOH on the mechanical properties of low-calcium fly ash expansive soil were studied through expansion test, unconfined compressive strength test, environmental scanning electron microscope test and X-ray diffraction (XRD) phase analysis tests, and the soil microstructure and pore formation characteristics were analyzed and quantified. Based on XRD test, the change characteristics of mineral composition in soil were analyzed, and the mechanism of action between alkali excited fly ash and soil was revealed. The results show that alkaline activator can enhance the activity of fly ash, accelerate the hydration reaction, accelerate the development of early strength and improve the later strength. With the increase of alkali activator, the unconfined compressive strength of fly ash improved expansive soil first increases and then decreases, and 10% NaOH is the best content. The types of pores in the soil are complex, and the gel produced by the microscopic discovery of alkali-stimulated modified soil can fill the pores, and bond with the soil to reduce the pores. As an activator, NaOH can effectively improve the activity of fly ash and improve the basic properties of expansive soil.