The Mg-5%Hg-1%Ga alloy was prepared by melting-casting. Aging at 573-773 K for different time were carried out to ensure the fine dispersion of Mg3Hg compounds. Scanning electron microscopy (SEM), emission spectrometry (ESA) and X-ray diffraction (XRD) were employed to characterize the morphology and distribution of the secondary phase Ma3Hg. The electrochemical and corrosion behaviors of the alloy in 3.5wt% NaCl solution was studied by means of potentiodynamic, galvanostatic and electrochemical impedance spectroscopy (EIS) measurements. The relationship between the Mg3Hg compound and the electrochemical properties of the Mg-5%Hg-1%Ga alloy was discussed. The results demonstrate that when the aging time increases from 15 min to 200 h, the intergranular eutectic of α-Mg and Mg3Hg changes into dispersed Mg3Hg particles in α-Mg matrix. This transition increases electrochemical activity of Mg-5%Hg-1%Ga alloy due to the decreased concentration polarization and charge transfer resistance in the faradaic reaction. The electrochemical homogeneity of the microstructure increases the corrosion resistance of the Mg-5%Hg-1%Ga alloy.