Cast-in-situ underground concrete structures suffer from both sulfate and sulfatechloride attack and are also affected by alternate dryingwetting conditions. Concrete specimens were manufactured and immersed in sulfate and sulfatechloride solutions. The wettingdrying tests of concrete specimens were also considered. The size and weight changes were recorded and the compressive strength tests were conducted. The sulfate concentrations in concrete after different immersion period were measured. The SEM, XRD, and EDS were also conducted and analyzed to reveal the degradation mechanism of concrete. The results show that the existence of chlorides accelerates the degradation of concrete caused by sulfate attack and the situation becomes ever worse after wetdry cycles. The strength of concrete specimens immersed in sulfatechloride solutions, experiencing wetdry cycles, suffers from remarkable decline. Chloride has a positive effect on accelerating the diffusion and the accumulation of sulfates in specimens subjected to composite attack, especially in the condition of alternative wetdry cycles. The degradation of castinsitu concrete due to sulfate attack can be accelerated by the existence of chlorides. The chloride ions, in the condition of alternative wetdry cycles, accelerate the diffusion and the accumulation of sulfate ions in concrete.