Abstract: In order to simulate the transient formation of a keyhole during the welding process accurately, the gas-liquid interface of the keyhole was traced by means of the level set method, the solid-liquid phase transition process was disposed by the mixture model, and the gas, liquid and solid coupling model was established. The factors, such as surface tension, buoyancy, recoil pressure, friction between solid and liquid, latent heat, convection and radiation were taken into account in this model. The dynamic process of the keyhole and the behavior of the metal vapor inside and outside the keyhole were obtained by numerical calculation. The simulation results show that the morphology of the keyhole was transient, had convex deformation in anterior and posterior and became stable gradually. The size of the keyhole was about 1mm. The keyhole and the metal vapor interacted each other. The greatest vapor velocity was 5.3m/s. A modified "sandwich" novel method was used to conduct laser welding experiment verification. The simulation results matched well with the experimental results. The results show that the level set method to track the free surface of a keyhole in laser deep penetration welding has good adaptability and can provide the theoretical basis for the study of keyhole.