It is of great significance for the repair of concrete in marine engineering to reveal the effects of slag on the degradation mechanism of ordinary Portland cement-calcium aluminate cement-gypsum (OPC-CAC-C S-) ternary binder under the combined action of multiple erosive ions and dry-wet cycles. In this study, mechanical property, mass change, Cl− content, hydration products and morphology were investigated. The results show that the compressive strength, the Cl− erosion resistance and the stability of hydration products of the ternary binder were significantly improved by the partial substitution of slag, especially in the erosion environment containing SO42− and Mg2+. As the aluminum phase was sufficient when slag added, the combination rate of Cl− improved by the presence of SO42−, while slightly decreased by Mg2+ at early ages. Compared with corrosive solution including only Cl−, the presence of SO42− and Mg2+ reduced the generation of Friedel's salt. Moreover, the stability of ettringite was obviously enhanced by SO42−, while slightly reduced by the additional Mg2+ in corrosive solution.