Wet-dry cycles of fly ash (FA) and cellulose fiber double-doped concrete with sulphate in Nazi Gorge water conservancy pivot project were investigated. The durability value (D), a durability index that can comprehensively reflect the mass, compressive strength and splitting strength of concrete were established based on an entropy weight method. The effects of sulphate mass fraction and fly ash admixture on the D were analyzed. A grey model-back propagation (GM-BP) neural network was constructed to reveal the time-varying law of the D. The results show that the D can reflect the effects of sodium sulphate mass fraction and fly ash admixture on the durability of concrete. At 180 day, the D loss is proportional to sodium sulphate solution mass fraction within 10% (mass fraction). The GM-BP neural network can predict the time-varying law of the D under salt intrusion-wet and dry cycles more accurately than the GM.