In order to solve the problems of large number of gas holes, coarse grain size and poor mechanical properties of traditional aluminum alloy welded joints, laser-arc hybrid welding experiment assisted by ultrasonic vibration was carried out by taking 5083-O aluminium alloy as the research object. The effect of ultrasonic vibration on the number of pore, microstructure and tensile strength of aluminium alloy weld was studied. The mechanism of the effect of ultrasonic wave on pore discharge and microstructure refinement in welding pool was also discussed. The results show that, the number of gas holes in the weld seam of ultrasonic assisted welding decreases significantly. It is mainly attributed to the reduction of hydrogen concentration in aluminium alloy melt by ultrasonic cavitation. It also promotes the rapid escape of bubbles. The cavitation effect and acoustic flow effect of ultrasound change the pressure, temperature and flow state of melt. The crystallization condition of the molten pool is changed. The grain structure of the weld is refined by increasing the nucleation rate and breaking dendrite. The average tensile strength of weld increased from 242.9MPa to 270MPa after ultrasonic vibration is applied. The fracture location occurs in the heat-affected zone. It is mainly due to the decrease of porosity and the refinement of structure in the weld zone. This study is helpful to understand the forming mechanism of defects and improve the strength of joints in the welding process of aluminium alloys.