Abstract: With Nb sheet and Cu sheet as composite interlayer materials, the vacuum diffusion welding was carried out on TC4 titanium alloy and 15-5PH stainless steel on a Gleeble-1500 thermomechanical simulator. The tensile strength of the joint was tested. The morphology at the interface and of the tensile fracture of the joint was observed, and the micro area composition was analyzed. The results show that the composite interlayer of Nb and Cu was helpful to restrain the interdiffusion of Ti, Fe and Cr, which came from titanium alloy and stainless steel. The strong bonding at stainless steel/Cu, Cu/Nb and Nb/titanium alloy interfaces was formed by atomic diffusion. A small amount of fine Nb-Fe intermetallic compound was formed at the Cu/Nb interface, which had little effect on the tensile strength of the joint. The maximum tensile strength reached 540 MPa. During tensile fracture the cracks propagated through the Cu layer, the Nb layer and the intermetallic compounds.