In the process of high power fiber laser welding of SUS 304 stainless steel plate, the dynamic pool endures imbalanced force and is very unstable. The pool is prone to droping down. The pool is droping down, which makes a flow accumulation at the bottom of the specimen, and a root humping forms. By using highspeed camera and“sandwich”new welding method, the pictures of the flow of the molten metal from different angles are taken and the longitudinal keyhole and the dynamic pool are observed. The results show that the molten metal flow is not continuous in the keyhole, the moving downward shelf on the front of the keyhole wall and the pressure of metallic vapor inside the keyhole lead to a plurality of droplet in the bottom of the pool. The surface tension and the flow of melt pool cause the backward flow of the molten metal. With the accumulation of molten metal, the root humping is formed. The root humping influences the flow state of melt pool, which results in the undercut on the weld surface.