Transient liquid phase (TLP) diffusion bonding of TC4 titanium alloy was realized using an electrodeposited Ni/Cu layer as interlayer. The effects of Cu interlayer thickness on interfacial microstructure and mechanical property of TC4 joint were studied by scanning electron microscopy, energy disperse spectroscopy and X-ray diffractometer, and the reaction mechanism was clarified by Ti-Cu and Ti-Ni binary phase diagram. The results show that the typical interfacial microstructure of the joint is TC4/α-Ti+Ti2(Cu, Ni)/TC4, and Ni element exists in the form of solid solution. With the increase of the thickness of electrodeposited Cu layer, the width of diffusion layer and welding seam increased, and the voids in the center of the joint disappeared. Continuous Ti2(Cu, Ni) intermetallic layer appeared in the reaction layer and its width increased gradually. The tensile strength of the joint increased first and then decreased, reaching the maximum value of 500 MPa when the thickness of Cu layer was 15 μm. Fracture analysis showed that all of the bonded joints fractured at the welding seams in the form of cleavage modes.