The cohesive energies, Fermi energies and local density of states (LDOS) are calculated by the first-principles based on pseudopotential plane wave method in this paper to investigate the physical nature of corrosion of Pb-Mg-Al alloy. The mechanism of electrochemical corrosion is analyzed according to the calculated electronic structure parameters. The results show that the stable phase in Pb-Mg-Al alloy is Mg17Al12>Mg2Pb>Mg. The Fermi energy (Ef) values of these phases with Ef(Mg)>Ef(Mg2Pb)>Ef (Mg17Al12) indicate that Mg is most likely to lose electrons while Mg17Al12 is difficult. LDOS result reveals that Mg and Mg2Pb phases are unstable compared with Mg17Al12 in the same external conditions, they are more likely to lose electrons and easier to corrod. The difference in Fermi energy between different phases in Pb-Mg-Al alloy forms the electrodynamic force of the electrochemical corrosion, which leads electrons to flow from the Mg and Mg2Pb phases with higher Fermi energy to Mg17Al12 phase with lower Fermi energy, further to corrode in Pb-Mg-Al alloy.