<p>Ultrasonic spot welding, as a multi-physical process coupling the thermal and mechanical behaviors, has been utilized to join Cu/Cu joint. The formation process, temperature distribution and plastic strain during ultrasonic spot welding as well as the fracture behavior for the Cu/Cu joints are not clear. Thus, a 3D thermo-mechanical finite element model for ultrasonic spot welding of Cu sheets considering the air convection, ultrasonic softening, friction and plastic deformation heat, and real dimensions was established. This was validated by comparing the simulated temperature and effective thickness with experimental results. By analyzing the simulation results on the temperature and strain, the mechanisms of microhardness evolution were clarified. A critical strain value at the welding interface for obtaining an excellent metallurgical bonding interface was estimated. Moreover, a tensile test model considering the effective thickness and bonding density was established, and the distribution and variation of the stress during the tension process were obtained. The joint fracture mechanism was systemically demonstrated by combining the simulation and experiment results.</p>