In order to reduce the distractions of plasma plume in laser welding and expand the applications of high power laser welding, the characteristics of bead formation and plasma plume in fiber laser welding under vacuum are studied. The results show the sound bead formations and an increase in penetration depth are obtained. The penetration depth under vacuum is approximately three times as deep as that under atmospheric pressure when the welding speed is 0.5 m/min. The experiment results also show the penetration depth increases drastically with the pressure decreasing under a low welding speed and a long negative defocused distance is suitable for a deep penetration and a sound weld beam formation under vacuum. The camera observations show the plasma plume is suppressed under vacuum. The suppression of plasma plume under vacuum is a reason for the increase in penetration depth under vacuum.