Corrosion resistance is an important fact to be considered in practical engineering application of Al-Mg2Si alloys, besides the mechanical properties. So in the paper, corrosion behavior of Al-20%Mg2Si with/without Al-3%P master alloy in 3.5% NaCl solution was investigated. For the Al-20%Mg2Si alloy, Mg2Si acts as anode and corrosion starts from its surface at the initial stage of corrosion process. As the corrosion proceeds, electrochemical polarity converts between Mg2Si and α(Al), which results in the corrosion of Al matrix with the formation of pits. Moreover, after addition of Al-3%P master alloy, Mg2Si transforms from enormous dendrite to fine and uniformly distributing polyhedron, which inhibits the corrosion pits from propagating in Al matrix and enhances the corrosion resistance of Al-20%Mg2Si.