Abstract: Environmental dredging is an important means to restore heavy polluted water. Large quantities of residual water with contaminants are produced during the dredging. Residual water treatment is one of important steps to control second pollution of environmental dredging. By simulating and full-scale experiments, different ways of adding flocculants were investigated. Several flocculants that adapt to residual water treatment were selected and compounded. Experimental results show that adding flocculants both to the sludge pipe and to the discharge outlet of disposal site can attain discharge standard of residual water. It is easy for adding flocculants to the discharge outlet to control discharge quality. The expense of flocculants to be added to discharge outlet is one-tenth of that to be added to sludge pipe, but constructing the facilities for coagulation and sedimentation is indispensable. Comparing to the effect of different flocculants, the results show that composite flocculants are preferable, the expense of residual water treatment is low, and residual SS is small.