To improve the durability of concrete exposed to sulfate attack in saline soils of Northwest China, fly ash was used as an equal-volume replacement for natural fine aggregate to prepare fly ash fine-aggregate concrete (FAFC). The mass-loss rate, compressive strength, relative dynamic modulus of elasticity, and air-void parameters were measured during sulfate wetting-drying cycles. The deterioration mechanism was interpreted by linking air-void evolution with sulfate-reaction pathways. A comprehensive durability index was established using the entropy-weight method, and a GM(1,1) model was developed for service-life prediction. The results indicated that CFA-20 exhibited the best overall performance. After 150 cycles, its compressive strength and relative dynamic modulus of elasticity were 45.62 MPa and 81%, respectively, while its sulfate-resistance grade K_S reached 133. Its comprehensive durability index was 1.34 times that of CFA-0, and the slowest deterioration rate was observed. The predicted scenario-based service life of CFA-20 was 10.88 years, which was 1.69 times that of CFA-0. Considering mechanical performance, multi-index durability, and predicted service life, a 20% replacement ratio is recommended. The results provide a basis for the durability design of concrete in sulfate-rich environments.