The melt flow in the rear part of molten pool affects the formation of weld defects in the high speed laser welding. A weld shaping device is designed in this paper to assist the high speed laser welding of Q235 steel. The three-dimensional numerical simulation model is developed to investigate the influence of weld shaping device on melt flow. In this model, the dynamic mesh method is used to describe the interaction of weld shaping device with melt metal during the high speed laser welding process of Q235 steel. Both of the weld morphologies and flow fields of molten pool under different process parameters conditions are calculated. The results show that the melt metal with high backward flow velocity changes the flow direction towards the front part of molten pool after colliding with the wall of weld shaping device, which reduces the accumulation of melt metal in the rear part of molten pool. Under the different weld shaping device angle conditions, it is found that the backflow velocity of melt metal in the rear part of molten pool increases with the decrease of weld shaping device angle. The high backflow velocity of melt metal from the rear part of the molten pool to the front of the molten pool can effectively suppress the hump and undercut defects in the high speed laser welding. The above analysis results indicate that the designed weld shaping device can be efficient for improving welding quality in the high speed laser welding.