Herein, the dynamic characteristics of the laser deep penetration welding plume were observed using high-speed camera, and plume formation reasons were analyzed. Results show that plume was divided into the periodic oscillating plume at the bottom and narrow plume, which was similar to a laser-focused form. When laser-induced evaporation steam on the front wall of the keyhole was close to the keyhole orifice, the swinging angle of the periodic swing plume at the bottom was the largest, whereas that of the narrow plume was smallest. Moreover, the intensity of the narrow plume increased with increasing defocusing amount (or the diameter of the keyhole in deep penetration). Further analysis showed that the formation of the periodic oscillating plume at the bottom was related to the steam generated from laser-induced evaporation on the front wall of the keyhole, and a large number of particles were observed in the eruption process of the bottom-oscillating plume. The formation of the narrow plume was related to the particles in the plume entering the beam and being heated by the laser beam.