It is necessary to make the hull potential uniform in order to restrain the characteristics of underwater electric field. A ship hull model in air-sea two-layer medium was established through boundary element method. By changing the number and distribution of sacrificial anodes in passive cathodic protection system, the potential of ship hull and underwater electric field under different protection conditions were calculated quantitatively, and an experiment in laboratory was carried out. Simulation and the experiment results show that the action distance of protection current between anode and corroded area can be shortened by reasonable design of cathodic protection system, which can make share the anticorrosive function of the same area with multiple anodes and can reduce the intensity of protection current output from single anode to improve the uniformity of hull potential, and can restrain the objective of underwater electric field while achieving anticorrosive effect.