The weld formation quality is highly related to the welded joints performance during the high speed laser welding. The humping, undercut, pore and other welding defects induced by the instable dynamic behaviors of molten pool are often found in the high speed laser welding. A 3D numerical model is proposed to investigate the molten pool and weld formation characteristics in high speed oscillating laser welding of aluminium alloy. The developed numerical model is validated by the experimental results. The evolution process of the molten pool and weld in the high speed laser welding are analyzed based on the numerical calculation results. The effects of the oscillating frequency and amplitude of laser beam on the weld formation process are discussed.  It is found that the weld morphologies with discontinuities are gradually formed with the increase of welding speed. The effect mechanism of the instable molten pool on the weld formation process in the high speed oscillating laser welding of aluminium alloy has been revealed. The results demonstrate that the proposed model can provide the new guidance for improving the instability of the molten pool and hence achieve the weld formation with high quality during the high speed oscillating laser welding of aluminium alloy.