Abstract: AZ31B magnesium alloy was successfully joined by ultrasonic assisted contact reactive soldering technology with Zn as the interlayer. The effect of ultrasonic vibration time on the microstructure and mechanical properties of the joint was studied. The microstructure of the joint after ultrasonic treatment for 3 s is composed of diffusion zone, eutectoid structure and Mg-Zn intermetallic compound. With the increase of ultrasonic treatment time, the shape of Mg-Zn intermetallic compounds gradually changed from long strip to granular, and the number gradually decreased. With the extension of ultrasonic action time, the weld width increases first and then decreases, and the joint strength increases. The highest strength of the joint after ultrasonic treatment for 9 s was 57.5 MPa. Fracture analysis showed that the fracture mode of the soldering joint was mixed fracture. With the increase of ultrasonic time, the fracture position of the joint gradually shifted from Mg-Zn intermetallic compound to α-Mg + MgZn eutectoid structure.