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The microstructure of Mg-Al-Zn-xCa (x=0%, 1.25%, 1.74%, 2.53%) alloy before and corroded has been characterized by XRD, SEM, TEM and EDS. And the corrosion behavior has been studied by weight-loss, electrochemical method and SPM. Results shown that the phase composition consists mainly of α-Mg, β-Mg₁₇Al₁₂, Al₂Ca and (Mg, Al)₂Ca. With the increasing of Ca, the size of dendritic arm decreased, and the scale and number of Mg₁₇Al₁₂ phase decreased gradually. The shape of Al₂Ca phase transformed from bonelike to network on grain boundary, and (Mg, Al)₂Ca phase increased gradually. The result of corrosion behavior testing shown that the content of α+β eutectic reduced by addition of Ca, and therefore led to the corrosion channel narrowed and the proportion decreased. The formed Al₂Ca cathode phase inhibited the hydrogen precipitation process and adding Ca led to an increase in anodic corrosion potential. The potential differences of Al₂Ca and α-Mg are lower than β-Mg₁₇A_l12and α-Mg.