The chloride migration test was used to study the transport phenomenon of chloride ion in concrete. Concrete specimens with different replacements of cement by slag (0%, 10%, 20%, 30%, 40%), and water-binder ratio (0.35, 0.45, 0.5) were cast and tested. Test results indicate that slag can reduce the chloride permeability of concrete when high water-binder ratios (0.45, 0.55). But they are no difference when concrete with low water-binder ratio (0.35).