Abstract: In activating flux PAW welding, the undercut and fatigue property were studied.The results show that the increase of welding speed can improve the welding efficiency, but the undercut appears.The deposited metal in activating flux PAW welding increases, the arc force strengthens, the arc voltage is higher, the arc stiffness strengthens and the arc is much more stable, which would make the deposited metal flow to the toe zone.The deposited metal would fill up the toe zone.Compared with the specimen without activated flux, the fatigue strength of specimen with activating flux has been increased by 60% at most.The activating flux can make the undercut reduce or vanish and make the fatigue property increase.