Abstract: Aluminum/magnesium alloys have increasingly important applications in the lightweight design and manufacturing of launch vehicles due to their excellent mechanical properties. The bodies and body cover panels of new energy vehicles contain numerous three-layer plate spot welding structures. In view of the common problem of difficult welding of dissimilar metals, the spot welding of Al/Mg/Al three-layer plates was realized by using ultrasonic welding technology. Meanwhile, a thermo-mechanical coupled analysis model of the ultrasonic spot welding process was established, and the accuracy of the model was verified. The results indicate that with the increase in welding energy, the tensile shear strength of the joints first increases and then decreases. The failure modes of the joints are all interfacial failures, and the maximum tensile load reaches 683.2 N. The interfacial bonding mechanism is mainly the mechanical interlocking formed by plastic deformation, and no metallurgical reaction occurs. The distributions and evolution laws of the temperature field and the stress field under different parameters are obtained through the numerical model, and the maximum temperature during the welding process can reach 600 °C. The research results provide new ideas and theoretical support for the research on spot welding structures of dissimilar metal three-layer plates