Abstract: The composite casting billet of Cr5/45 steel was prepared by electroslag remelting composite method. The longitudinal section of the sample after dissection was studied. The equilibrium phase diagram of the two materials was calculated by Thermo-Calc, and the heat treatment process of 900 ℃ quenching+530 ℃ tempering was adopted. The element transition, microstructure, tensile and shear properties of the as-cast and heat-treated specimens were tested. The results show that the heat treatment can promote the diffusion of Cr at the bonding interface of the bimetal composite billet of Cr5/45 steel,and the thickness of Cr transition zone increases from 70.1 μm to 91.0 μm. Tempered sorbite is found on the side of Cr5 steel in the heat treated sample. 45 steel side is tempered martensite, and secondary cementite precipites at the grain boundary. In addition, the interfacial tensile strength and shear strength of the samples after heat treatment are 870 MPa and 1588 MPa,respectively, and the tensile fracture occurs on the side of the 45 steel,which fully indicates that the bimetal interface is well bonded.