The composition, morphology and the long-term corrosion resistance in sodium chloride solution of zinc phosphate coating on AZ31B magnesium alloy were investigated using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, hydrogen evolution and potentiodynamic polarization curve experiments. The results indicate that the phosphate coating consists mainly of Zn 3 (PO 4 ) 2 · 4 H 2 O, Zn 2 Mg(PO 4 ) 2 and metallic Zn. The variations of the hydrogen evolution, i corr and corrosion morphologies indicate that the corrosion process of zinc phosphate has three stages. At the first stage, the corrosion resistance of the phosphate sample is higher than that of the uncoated sample. At the second stage, the corrosion rate basically remains constant. At the third stage, a Zn – Mg galvanic cell and galvanic corrosion are formed between zinc and the AZ31B substrate, and the corrosion resistance of the phosphate sample is lower than that of the uncoated sample.