Abstract: Ultrasound-promoted transient liquid phase bonding (U-TLP) can achieve fast connection of difficult-to-wet materials in the atmosphere. It is a high-quality, high-efficiency, low-cost advanced connection method. This paper uses AZ31B magnesium alloy as the substrate. Pure zinc is the middle layer, and the evolution law of the microstructure of the bonding zone under ultrasonic irradiation is analyzed. The formation mechanism of Mg/Zn/Mg U-TLP interface defects is investigated. The results show that the microstructure of the bonded zone is composed of fine Mg-Zn eutectic with the extension of ultrasonic time. The liquid phase structure consisting of alternating MgZn and Mg7Zn3 phases gradually transforms to the fully diffused α-Mg phase, and the joint shear strength increases. However, when the ultrasonic time is increased to 60s, due to the loss of the liquid phase, pore defects appeared along welding seam. The joint shear strength is 55 MPa, which is 64.7% of the base material compressive shear strength. Using the secondary ultrasonic optimization process during the cooling process, the joint shear strength increased by 17%.