Abstract: The transient behavior of keyhole in laser deep penetration spot welding was studied by method of numerical simulation. A numerical model of two-dimensional transient keyhole in laser spot welding was established. The evolution and transient behavior of keyhole in the process of both continuous and pulse laser spot welding were analyzed respectively. The various factors that affected the growth of keyhole were studied by applying different calculation conditions. The results showed that depth of the keyhole increased with the accumulation of heat input and the keyhole oscillated constantly in the continuous laser spot welding. However,the evolution of the keyhole was periodic and consistent with the pulse period in the pulse laser spot welding. When the laser was turned off,the depth of the keyhole decreased or even closed. When the laser was turned on,the keyhole increased quickly to the depth before the laser was turned off. The recoil pressure was the necessary factor to form the keyhole,while the surface tension was the force to hinder the formation of the keyhole. With the increase of viscosity,the width of weld pool decreased and the depth of keyhole and weld pool increased.