Abstract: A moving free interface is formed by interaction between welding arc and weld pool.How to deal with the interface is the key to realize coupling interaction between welding arc and pool.Basing on 3D united mathematical model,interaction between TIG welding arc and pool was numerically simulated in this paper.And the theory of interaction between arc and pool was revealled.The result showed that shape of the calculated weld pool was in agree with that of the experiment.