The invention belongs to the field of steel material manufacturing, and in particular relates to a method for manufacturing Martensitic-ferrite plastic mould steel. The steel comprises the following elements in percentage by mass: less than or equal to 0.27 percent of C, less than or equal to 0.40 percent of Si, less than or equal to 1.00 percent of Mn, less than or equal to 2.00 percent of Cr, less than or equal to 0.59 percent of Mo, less than or equal to 0.30 percent of Ni, less than or equal to 0.030 percent of P, less than or equal to 0.030 percent of S, and the balance of Fe and inevitable impurities. The method for manufacturing the plastic mould steel comprises the following steps of: heating the steel in a heat treatment furnace to be between 720 and 830 DEG C, after preserving the heat for 10 to 50 minutes, quenching the steel into oil, cooling the steel to the room temperature, and then tempering at the temperature of between 100 and 600 DEG C. The steel consists of the tempered Martensitic-ferrite and carbides which are distributed uniformly, wherein the tempered Martensite is 65 to 95 percent based on the total volume of the steel; and the average diameter of the carbide particles is between 0.01 and 1.5 mu m. The rigidity of the steel is 30 to 40HRC, and impact toughness is 30 to 80J. The plastic mould steel prepared by the method has the advantages of high strength, high elongation, high impact toughness, high polishing performance and low cost.