Abstract: In order to realize the low temperature direct joining of TC4/AZ31 in the atmospheric environment, the surface modification of TC4 was carried out by ultrasonic assisted pre-plating, and then the ultrasonic assisted brazing method was used to realize the bonding between the modified TC4 and AZ31, which solved the problem that Ti and Mg elements could not be directly brazed due to their insolubility and non-reaction. Firstly, the dynamic process of zinc plating on TC4 surface under ultrasonic action was simulated by ABAQUS, and the optimal ultrasonic-assisted pre-plating parameters were obtained : when the hot-dip temperature was 500 ℃, the diameter of the sonotrode was 50 mm, and the amplitude was 10μm, the maximum flow rate of TC4 surface in the zinc bath was 0.23 m/s, and the average surface sound pressure was 13.1 MPa. Secondly, ultrasonic-assisted hot-dip galvanizing was performed on TC4 to form a uniform ( TiZn₃, TiZn₃ + TiZn₁₆, η-Zn ) coating on its surface. Finally, ultrasonic-assisted brazing was used to join TC4/AZ31, and the effects of brazing temperature and ultrasonic time on the microstructure and properties of heterogeneous joints were studied. The low temperature joining of galvanized TC4 and AZ31 was realized at 230 ℃. The brazing seam was composed of ( TiZn₃, TiZn₃ + TiZn₁₆, η-Zn ) layer, ( β-Sn + η-Zn ) layer and Mg₂Sn. With the increase of ultrasonic time, the thickness of Mg₂Sn increases, and the overall width of the brazing seam decreases. The shear strength of the joint is controlled by the thickness and morphology of the brittle Mg₂Sn layer. When the ultrasonic time is 5 s, the thickness of Mg₂Sn is 6.7 μm, and the average shear strength of the Ti/Mg dissimilar joint reaches 47.7 MPa.