Mg/Ti dissimilar metals were welded by ultrasonic welding. The effects of different welding energy on the interfacial peak temperature, interfacial morphology, interfacial atom diffusion degree and mechanical properties of the joint were investigated. It is shown that the overall interface of Mg/Ti is relatively straight during the welding process, the local interface has small fluctuations, no cracks, lack of fusion and other defects are found, and no obvious reaction layer is observed. The interfacial peak temperature ,the thickness of the atomic diffusion layer , the connection zone and mechanical properties of the joint increase with the increase of welding energy. When the energy reaches 2000J, the magnesium side base material shows welding cracks. The joint fracture mode is divided into interface fracture and button fracture. The fracture location occurs at the magnesium side diffusion layer area and the magnesium side non-diffusion layer area. Scanning electron microscopy and X-ray diffractometry analysis showed that there was no obvious intermetallic formation in the Mg/Ti interface region.