Physical models of cathodic protection (CP) for copper protected by 35# steel and for 35# steel protected by aluminum sacrificial anode respectively in seawater were built in this study. Weak form of Laplace equation was deduced to make finite element method (FEM) numerical calculation conveniently. Then, potential distribution of each physical model was computed by FEM following with measured experiments for validation. Typical FEM simulated data along X-axis and Y-axis of physical models agree well with measured experimental data. For 35# steel protected by aluminum sacrificial anode models, normal cathodic protection, over-protected (OP) and under-protected (UP) models were built, respectively. The results demonstrated that it should be feasible of CP for copper by 35# steel and for 35# steel by aluminum sacrificial anode respectively, and FEM could afford well forecast for CP design in engineering.