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Corrosion resistance of cerium conversion film on Mg-Gd-Y-Zr alloy was investigated by volumetric method. The optimum deposition parameters were obtained by orthogonal table L9(43), i.e. pH value 10.0, electrochemical deposition time 30 min, accelerating agent concentration 0.05 mol/L, and temperature 25 ℃. The influence degrees on corrosion resistance were in the sequence of pH > electrochemical deposition time > accelerating agent concentration > temperature. The corrosion resistance of cerium conversion film, chromated conversion film and Mg-Gd-Y-Zr magnesium alloy in 3.5% NaCl solution was compared by measuring the rate of evolved hydrogen. Results show that cerium conversion film improves the corrosion resistance of RE-Mg alloys remarkably.