The plate welding experiment is carried out by IPG YLS-600 fiber laser. The effect of plume is observed by a high speed camera, and measuring the weld width and penetration as the plume height is controlled by using the supersonic cross jet at different heights during bead-on-plate welding. The results show that the plume can be divided into two parts: a randomly fluctuating plume out of the keyhole and a slender plume similar to the focused laser beam. The weld penetration and width have an improvement of about 20% and a decrease of about 24%, respectively, as the rising of plume is controlled by applying a cross jet at 5 mm height during welding. And the stability of the welding process and the appearances of the weld seam are also greatly improved. Further analysis indicates that the effect nature of plume on the fiber laser is the effect of particles in plume on the fiber laser, which results in a significant effect on the welding process.