Combined attack of chloride and sulfate on cement based materials was investigated. The results show that, the mass change ratio is a exponential function of expansion ratio for the specimens during the process of sulfate attack. Chloride can decrease the risk of expansive damage of specimens under sulfate attack. Chloride inhibits transport of sulfate ions. Meanwhile，the chemical bond energy between sulfate ions and cement minerals is weakened, which results in decrease of corrosion products content. Simultaneously, the chloride ions preferentially react with C3A to produce the Friedels salt. The reaction product fills the capillary pore and decreases the pore size of specimens. The reaction capacity of sulfate ions with cement minerals is further restricted.