An external heat source was used to change the temperature distribution around the pin in the friction stir butt welding system of Al/Cu dissimilar metal to improve the mechanical properties of the welded joint. The results show that the external heat source increased the temperature at copper side obviously, which also increased the fluidity of weld joint. The change of temperature field decreased the content of CuAl and CuAl2
in the weld joint through XRD spectrums, and increased the content of copper particle. All of this resulted in the increase in the tensile strength of the weld joint and a uniform distribution of microhardness around the welding line.