Effect of heat treatment process on corrosion performance of plastic die steel P20 and mechanism for corrosion were studied by electrochemical test, electrochemical impedance spectrum test, weight loss test, neutral salt spray test, SEM, EDS and XRD. It is found that the steels with bainite/martensite quenched at 860℃ by air cooling or martensite quenched at 860℃ by oil cooling have better corrosion resistance than the steel with ferritic/martensitic quenched at 785℃ by oil cooling. After the samples with bainite/martensite or martensite are tempered at 450 and 620℃ respectively, martensite and bainite decompose and a lot of carbides precipitate, leading to a poor corrosion resistance. After electrochemical test, the outer layer of corrosion product on the surface of the samples is loose FeOOH and the inner layer is relatively dense Fe3O4. The anode reaction is mainly dissolution of Fe and the cathode reaction is generation of hydrogen. Pitting mainly occurs around inclusions in NaCl solution of 0. 5mol/L.