This work aims to study the influence of Mg element content on the structure and corrosion resistance of bridge cable wire, and to further improve the corrosion resistance of bridge cable wire on the basis of ensuring the mechanical properties of bridge cable wire. At about 450 ℃，hot dip plating method was used in steel wire surface and the Zn-Al-Mg coating was prepared. Electronic universal testing machine and torsion testing machine were used to investigate the strength and torsion performance of steel wire respectively. The microstructure of Zn-Al-Mg coating was addressed by scanning electron microscope (SEM) and energy spectrum (EDS). The phase composition of coating and corrosion products were analyzed by X-ray diffractometer (XRD). The coating corrosion resistance was represented by the neutral salt spray test. The corrosion behavior of Zn-Al-Mg coatings with different Mg content was studied by potentiometric polarization curve and electrochemical impedance spectroscopy. The results shows that the Zn-5Al-xMg (x=0.5, 1.0, 1.5) coating structure is mainly composed of ternary eutectic phase of Zn-Al-MgZn2, Zn-rich phase and a small amount of Al-rich phase. The addition of Mg element reduces the self-corrosion current density of the coating, promotes the generation of the protective corrosion product Zn5(OH)8Cl2.H2O, prevents the formation of the non-protective corrosion product ZnO, and effectively prevents the corrosion reaction from going into the alloy. Zn5(OH)8Cl2.H2O is a kind of dense and insoluble gel with good adhesion and low conductivity. When corrosion occurs, the coating will form a dense protective film on the surface of zinc alloy. Impedance and neutral salt spray test results also show that with the increase of Mg element content, the electrochemical ac impedance of Zn-5Al-Mg coating increases continuously, and the corrosion rate of neutral salt spray decreases continuously. The corrosion resistance of Zn-5Al-xMg coating achieved the best when Mg element content was 1.5wt%. And the corrosion resistance of Zn-5Al-1.5Mg coating was 2.4 times that of Zn-5Al coating.