To objectively evaluate the whole process of transport-reaction-damage of concrete under sulfate attack， a transport model considering porosity， tortuosity and critical damage degree was established based on the theory of crystallization pressure， volume expansion theory and Fick’s second law. The variable coefficient of sulfate transport was solved by alternating implicit difference method. The results show that the prediction model results are basically consistent with the experimental results， with the maximum error of 26.7%， it can effectively predict the diffusion law of sulfate ions in concrete. Analysis of influencing factors shows that the concentration of sulfate attack and the critical damage degree have a significant impact on the spalling thickness of concrete cover. Under the same critical damage degree， the spalling speed in 5% sodium sulfate solution is 51.8% higher than that in 3% sodium sulfate solution. At the same concentration of sodium sulfate solution， the critical damage degree increases from 0.80 to 0.95， and the spalling thickness decreases by 42.9%.