When we construct the breakwater using dredged soil improved with converter slag, it’s important to manage the prevention of pollution, the quality control of improved soil strength, the stability of the improved soil against waves. In this study, we determined the relationship between Suspended Solids (SS) and the mixing rate of converter slag, the strength of improved soil and integrated temperature in the curing period. We also studied the critical friction velocity against waves, the defect rate and the coefficient of variation of the strength of improved soil with converter slag.