Abstract: AZ31B magnesium alloy was successfully joined via ultrasonic-assisted contact reactive brazing, with Zn serving as the interlayer. The effect of ultrasonic vibration duration on the microstructure and mechanical properties of the joints was studied. The microstructure of the joint subjected to 3 s of ultrasonic treatment consists of a diffusion zone, an eutectoid structure, and Mg-Zn intermetallic compounds. With the extension of ultrasonic treatment duration, the morphology of Mg-Zn intermetallic compounds gradually transforms from long strips to a granular form, and their quantity decreases gradually. With the extension of ultrasonic treatment duration, the weld width increases first and then decreases, and the joint strength increases. The joint subjected to 9 s of ultrasonic treatment achieves the maximum strength of 57.5 MPa. Fracture analysis shows that the fracture mode of the brazed joints is mixed fracture. With the increase of ultrasonic treatment duration, the fracture location of the joints gradually shifts from the Mg-Zn intermetallic compounds to the α-Mg + MgZn eutectoid structure.