The corrosion inhibition behaviors of sodium phosphate (SP) on AZ31 magnesium alloy in 3.5 wt% sodium chloride solution were investigated by electrochemical impedance spectroscopy (EIS), potentiodynamic polarization curve, static weight-loss test and scanning electron microscope (SEM). The results indicate that SP protects AZ31 magnesium alloy from corrosion in 3.5 wt% sodium chloride solution, and the inhibition efficiency increases with higher SP content. When the SP concentration is 1.0 g/L, the inhibition efficiency is up to 81.5%. Moreover, according to the results of SEM, it is found that the corrosion inhibition of AZ31 magnesium alloy by SP is attributed to the protective film containing Mg(OH)2 and Mg3(PO4)2 formed on the electrode surface. The compact film, which reduces the substrate contact with Cl-, thus inhibits the anodic reaction of the magnesium alloy.