By combining hydrological exchange with chemical self-purification, the seawater self-purification capacity to heavy metals was studied, and the calculated results show that the hydrological exchange ratio during spring tide is 32.0%, and is 5.4% during ebb tide; the self-purification capacities to Cu, Pb, Zn and Cd in one cycle are respectively 1.04 t, 7.64 t, 15.22 t and 0.78 t; the hydrological exchange period is 10 d; the self-purification capacity to Cu measured is 42 t/d in the Haikou Bay.