High power laser welding is as an ideal advanced technology for joining the thick steel plates with high efficiency in the industries. This paper proposes a 3D numerical simulation modeling method to investigate keyhole profile characteristics and the effect of keyhole evolution on the molten pool, and understand weld formation process and defect generation, including the welding phenomena such as swelling, column and spatter in the weld pool during high-power deep-penetration laser welding. To reveal the weld formation process, the gas layer, keyhole free surface, molten pool surface, and solid-liquid-vapor three-phase transformation are considered in the model. Based on the proposed model, the keyhole periodic variation profiles and molten pool evolution are calculated. The interaction between the keyhole and molten pool dynamic behavior and weld defects are also identified and discussed in details. The numerical simulation results are compared with the experimental and literature results and good agreements are achieved. It is found that the constriction and bulge formed on the rear keyhole wall is the key factor causing periodical change of keyhole inlet. The oscillating interface between the keyhole and molten pool is often accompanied by swellings and columns, which is the main factor for spatter formation. (C) 2018 Elsevier Ltd. All rights reserved.