Theoretical analysis on ignition-proof of AZ91D alloy added with Ce was studied. The effect of Ce on burning temperature was investigated. The burning temperature increases with increasing Ce by insulation work at different temperatures. The influence of Ce on maximum insulation work time was also studied. The maximum insulation work time increases with increasing Ce and decreases with the melting temperature. There is a tendency for Ce to concentrate on the surface of melti ng AZ91D alloy. Ce will react with MgO and O2. The resultant will fill the cavity of MgO film to form multifilm. So the density of surface film of liquid Mg will increase to prevent burning.