Abstract: With considering the coupled behaviors of weld pool and keyhole, a three-dimensional transient model of thermal process in keyhole plasma arc welding is established. The enthalphy-porosity technique is used to deal with latent heat and momentum loss during melting and solidification. The keyhole boundary is tracked by VOF method. The transient evolution of keyhole geometry from its establishment to its open status, and dynamic variation process of fluid velocity field in weld pool are computed. Keyhole plasma arc welding experiments were conducted to validate the predicted results. The results show that the magnitude of the simulated velocity corresponds to reality and the trend of the fusion line is close to the experiment.