The TC4 titanium alloy and 304 stainless steel are welded by laser-arc hybrid heat source with copper interlayer. Macrostructure and microstructure of cross sections, main compositions, mechanical properties and fracture positions of welding joints are examined. The results show that the transition zone is characterized by Ti-Cu intermetallic, solid solution of copper, and Fe-Cu eutectic mixture. With the increase of laser power, the thickness of Ti-Cu intermetallic near the Ti-Cu interface increases, which has harmful effect on mechanical properties. Fracture of joint mainly happens at the interface between titanium alloy and copper when laser power is higher. The mutual diffusion depth and density of elements, such as copper and iron near the interface between copper and stainless steel, also increase, which is beneficial to enhancing mechanical properties. Fracture of joint mainly happens at the interface between the copper and stainless steel when laser power is lower. The key point to achieve favorite welding joint is the control of Ti-Cu intermetallic and Fe-Cu eutectic mixture.