Abstract: Vibration amplitude is an important parameter of ultrasonic welding, but insights into the influence mechanism of vibration amplitude on ultrasonic welding are few. The effects of vibration amplitude on the process of Cu/Al ultrasonic welding were investigated in this study, including vibration amplitude distribution of specimens, interfacial temperature, intermediate phase distribution, materials plastic deformation, and properties of joint. The results show that under the welding pressure of 1575 N and welding time of 0.5 s, with the increase of welding amplitude, the vibration amplitudes of the upper specimen exhibit a linear growth, and the amplitudes of the lower specimen tend to show a near-linear growth. During the welding process, the higher the vibration amplitude, the higher the vibration amplitude of the lower specimen is. When the vibration amplitude is set at 25 μm, a Cu/Al joint is obtained with a maximum tensile-shear force of 3150 N. High vibration amplitude has increased the relative motion speed among the sonotrode, the upper specimen and the specimen, which has sped up the temperature rise in the welding interface and the plastic deformation of the material, thus accelerated the interfacial atoms diffusion and promoted the formation of high quality Cu/Al ultrasonic welding joints.