Abstract: The arc ultrasonic welding technology which uses welding arc as the ultrasonic source can refine the grain and improve the welding joint quality.To research the feature of arc ultrasonic propagating in molten pool and base metal,and its effect on the bond area,the photo-elastic system was set up.Welding piece and molten pool were simulated with optical glass and water channel respectively.With the directive property of arc ultrasonic field,the influence of arc ultrasonic on different zone of fusion area was studied.The results show that there are strong oscillating around the bond line and the mechanical effects of ultrasonic refine the structure of fusion area.The structure in the middle of bond zone is refined smaller than in the toe zone for the reason of the directive property of arc ultrasonic.