Abstract: The formation process of pure copper joints by ultrasonic welding is determined by interfacial plastic deformation and grain microstructure evolution. However, the understanding of the ultrasonic welding mechanism for pure copper is still unclear. A finite element model of pure copper by ultrasonic welding was established to investigate the welding temperature field and plastic strain distribution. Secondly, the simulated temperature and plastic strain results were combined with the theory of dynamic recrystallization. Finally, the dynamic recrystallization process and the growth of the grain during the welding were simulated by the finite element method and the cellular automata method, respectively. The results show that the irregular distribution of grains leads to the obvious difference between the plastic deformation of the material under the joint, on the welding interface, and above the base. The area where the workpiece comes into contact with the welding head and the base has generated an ultra-fine grain that suffers dynamic recrystallization. After a welding time of 0.19 s, the dynamic recrystallization is generated in the entire welding area, and the process of dynamic recrystallization of the upper workpiece is longer than that of the lower workpiece. The coarse grains are distributed at the copper side of the joint interface, while the finer grains are distributed at the welding interface. The distribution of simulated plastic deformation is basically consistent with the distribution of experimental Vickers’ hardness. In addition, the growth of simulated grains is basically in line with the test. These demonstrate that plastic deformation and grain microstructure evolution in pure copper by ultrasonic welding are successfully simulated.