Based on the relationship between deformation microstructures and grain orientations, three characteristic Cu single crystals were used to investigate the opposite effects of ultrasonic superimposed high-strain-rate on the dislocation motion during ultrasonic welding (UW). The results revealed that equiaxed dislocation cells and discontinuous dynamic recrystallization (DRX) grains dominated in the joint microstructures. Three Cu single crystal joints exhibited an isotropic trend in grain orientation, welding quality, and microscopic mechanical properties. The preferred dislocation behaviors and DRX modes were further analyzed by modelling the stored energy difference, indicating that high mobility of intra-granular dislocations and homogeneous dislocation motion induced by the ultrasonic excitation were the intrinsic factors contributing to the formation of isotropic microstructures and welding quality.