Some plastic mold steels after being processed and molded will deform dramatically during heat treatment, whereby the precision of molds cannot be ensured. Therefore, mold steel can be supply to market in pre-hardened form, which is easy for manufacturing plastic molds of high precision and meanwhile the production cost will be reduced. The pre-hardened plastic mold steel is first modified to acquire required service performance and then processed and directly put into use without final heat treatment, which can avoid deformation and crackles caused by heat treatment. Pre-hardened steel is most suitable for manufacturing complicated large-and medium-sized precision plastic molds.