YG20 and 42CrMo steel are connected at 1 150℃ and 1 200℃ for 1h respectively by using Ni/V/Ni as the composite intermediate layer by adopting the solid-state vacuum diffusion method, followed by an analysis of the microstructure and tensile strength of the diffusion layer at different temperatures by scanning electron microscope, energy dispersive spectrometer, X-ray micro area diffractometer and mechanical testing machine. The results show that (γFe, Ni) and (Ni, Co) solid solution are formed at the interface of YG20/Ni and Ni/42CrMo steel; meanwhile, carbon atoms diffuse from YG20 and 42CrMo steel terminals to the composite intermediate layer, subsequently forming carbidesin in the diffusion layer. An analysis has been made of the formation mechanism of composite intermediate phase, combined with the C-Ni-Nb isothermal cross section calculated by phase diagram thermodynamics and the experimental characterization of diffusion layer phase. The phase forming sequence from Ni to Nb end is NbNi3/NbC, Nb2C and Nb7Ni6. Due to the difference between the thermal expansion coefficients of NbNi3 and NbC, NbC can be easily peeled off from NbNi3 matrix, thus forming holes. As the connection temperature increases from 1 150 ℃ to 1 200 ℃, more NbC precipitates, resulting in the increase of holes, which reduces the tensile strength from 40 MPa to 24 MPa with an overall brittle fracture at the NbNi3+NbC layer.