Abstract: TC4 titanium alloy and 15-5PH stainless steel was vacuum diffusion welded with Ta+Cu as composite interlayer under conventional process (raise temperature to 1 000℃ at rate of 20℃·s^-1, holding for 1 200 s with pressure of 6.7 MPa, air cooling) and step process (raise temperature to 1 050℃ at rate of 20℃·s^-1, holding for 2 s, then lower temperature to 950℃ at rate of 10℃·s^-1 with pressure of 2.8 MPa, and holding at a pressure of 6.7 MPa for 1 200 s, air cooling). The microstructure and properties of the joint were studied. The results show that the composite interlayer could prevent the element diffusion between titanium alloy and stainless steel. The tensile strength of the joint under step process was 550 MPa, and was higher than 390 MPa under conventional process, which was related to the small and shallow Kirkendal diffusion cavity, small thickness of the composite interlayer and good banding quality of Cu/Ta interface.