Electrochemical property of Mg-based alloy and Al-based alloy sacrificial anode in seawater was studied by means of galvanic polarization method in this paper.Galvanic current of aluminum alloy and magnesium alloy sacrificial anode coupled with Q235 steel were tested,in natural seawater of Qingdao.And cathode protection potential was obtained at the same time.Then the anodic current efficiency was calculated.The experimental results showed that the variety in corrosion current of Al alloy sacrificial anode coupled with Q235 steel could be plotted to four steps,and of Mg alloy sacrificial anode just decreased monotonically.This special phenomenon was discussed with emphasis in the paper.