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The spreading and wetting behavior of Mg-Zn-Al solder under the action of flux and ultrasonic vibration was conducted in atmosphere. The ultrasonic vibration time was 1, 2, 3, 4s, respectively. The spreading morphology and microstructure of the solder was investigated by optical microscope. The results show that the Mg-Zn-Al solder is forced spread under the effect of ultrasonic vibration along the base metal. The impact wave induced by ultrasonic cavitation can break the oxide film on the surface of base metal and the solder, which can promote the solder to wet the base metal. The depth of the dissolved base metal is only 0.12mm. When the ultrasonic time is 2s, the spreading area reaches its maximum. The ultrasonic cavitation can break the columnar crystals of Mg-Zn eutectic phase and the dendrite crystals of <i>α</i>-Mg, which can refine the microstructure of the Mg-Zn-Al solder.