Sulfate saline soil in seasonally frozen regions is prone to engineering hazards such as salt heave, frost heave, thaw settlement, and dissolution-induced collapse, which seriously threaten engineering safety and durability. Corn straw ash (CSA), as a green solid-waste material, has shown good application potential in the improvement of sulfate saline soil. To investigate the macro- and micromechanical response characteristics and damage mechanisms of CSA-modified saline soil during the freeze–thaw process, various experimental methods were adopted to analyze the evolution of mechanical properties and microstructural characteristics of the modified saline soil during the freeze–thaw process. A microscopic damage constitutive model under the coupled action of freeze–thaw and shear loading was established to reveal the damage mechanism of the modified saline soil. Finally, a conceptual model was developed to clarify the action mechanism of CSA. The results show that the shear strength of CSA-modified saline soil first increases and then decreases during the freeze–thaw process. In the frozen state, the stiffness and deformation resistance of the modified soil are significantly higher than those of untreated sulfate saline soil, and its performance degradation after thawing is relatively slight. The specimen with a salt content of 2% and a CSA content of 15% exhibits the best performance. Under room-temperature and frozen conditions, the pore-size distribution of the modified saline soil tends to become more concentrated, the proportions of medium and large pores decrease, and the surface structure becomes denser. In the thawed state, the proportion of large pores increases, and detachment of CSA aggregates occurs on the surface of specimens with high CSA contents. The established microscopic damage constitutive model can accurately describe the complete deformation–failure process and damage evolution of CSA-modified saline soil under the coupled action of freeze–thaw and shear loading. The study indicates that the incorporation of CSA enhances the structural continuity and water stability of sulfate saline soil, reduces salt heave and frost heave, and restricts the growth of salt crystals and ice crystals, thereby improving the freeze–thaw resistance of the soil. The results can provide a theoretical basis and technical reference for the engineering application and stability evaluation of CSA-modified sulfate saline soil in seasonally frozen regions.