Abstract: Under the background of modernization construction and the “dual-carbon” strategy, solid-waste based sulphoaluminate cementitious materials (SW-SAC) demonstrate broad application prospects. However, most existing studies focus on preparation technology and conventional performance optimization, while insufficient attention is paid to durability degradation under sulfate attack, especially the coupling relationship between the initial hydration state of materials and their sulfate erosion response remains unclear. SW-SAC was selected as the research object. A 30-day accelerated degradation test was performed with 5% sodium sulfate solution (5% NS) to simulate a sulfate-rich erosion environment. Compressive strength tests, MIP, X-ray diffraction-Rietveld refinement (XRD-Rietveld), FT-IR, and SEM-EDS were utilized as characterization methods to reveal the degradation process of its mechanical properties, pore structure, hydration products and microstructures under 5% NS erosion. Meanwhile, fly ash (FA) was incorporated to explore its functional mechanism for improving durability. The results show that SW-SAC undergoing continuous hydration exhibits dynamic competition between structural damage and compensation under 5% NS condition. SW-SAC with an initial hydration degree of 73.10% contains 21.39% expansible ettringite (AFt) together with calcium sulfate. Partial phases develop into large-volume columnar crystals in 5% NS solution, leading to abrupt bursting of specimens. With prolonged erosion duration, AFt undergoes decalcification degradation to 4.96%, accompanied by the precipitation of composite sulfate transitional phases, and the porosity reaches its peak value of 52.85%. Subsequent continuous hydration and expansion tend to fill structural defects, allowing the compensation effect to gradually gain dominance. The improved durability induced by FA originates from physical filling and hydration-modulation effects. Physical filling increases matrix compactness and restricts the ingress of aggressive agents. Hydration modulation retards the initial hydration degree to 50.9% and reduces the content of expansible precursors to 16.83%. It also advances the time at which the compensation effect gains dominance by approximately 40%. This paper clarifies the coupling damage mechanism between the initial hydration degree of SW-SAC and degradation induced by 5% NS erosion as well as the optimization mechanism of FA, which can provide a theoretical basis for the early engineering application and service performance optimization of such cementitious materials in extreme environments.