The composite die steel was prepared by vacuum diffusion welding with the cold work die steel Cr12MoV and the alloy steel 40Cr. The composition, microstructure and property changes of the welded joints were studied experimentally, and the relationship between the microstructure and the property after the final heat treatment was analyzed. Based on the experiment results, the suitable welding and heat treatment process were approached. The results show that the good metallurgical bonding is obtained via welding at 1100oC for 30 minutes with a pressure of 20 MP and not more than 0.01 Pa in vacuum. A number of voids are found in the tensile fracture surfaces when the welding temperature is below 1100oC. The grains of 40Cr steel grow obviously when the welding temperature is higher than 1100oC. After quenching at 1050oC and tempering at 200oC, the better mechanical properties are obtained in Cr12MoV because fine carbides distribute evenly. At the same time, the mechanical properties of 40Cr do not decrease dramatically because even though high temperature quenching brings coarse grain size, the amount of lamellar martensite, which is hard and brittle, decreases a lot and the amount of lath matensite increases. After quenching and tempering treatment, the bending strength of the composite die steel reaches 73% of that of Cr12MoV when treated by conventional processing. In the course of welding, carbon migrates from 40Cr to Cr12MoV, which results in a decrease in hardness at the interface of 40Cr.