Typical welding defects of the high speed laser welding process have a close relationship with the melt flow of molten pool. The investigation of melt flow behavior can help to further understand the formation mechanism of defects in the high speed welding process. In this study, fiber laser is used to weld austenitic stainless steel sheet. The effect of welding speed on melt flow behavior has been investigated. The surface flow behavior is studied by high speed visual detection system. In addition, the inner flow is studied through Ti tracer method. The results show that with the welding speed increasing, the flow distance of surface flow from keyhole to rear goes up, the molten metal in the middle of molten pool moves to the bottom and rear of pool with a longer distance. The effect of the welding speed on melt flow near the edge of the molten pool is small. It can be accounted that, with the welding speed increasing, the angle between vapour plume and pool surface decreases, the horizontal component of the vapour force will improve, which causes the backward driving force to increase and the flow distance of the surface flow to increase.