Abstract: Based on the theory of heat transfer and thermal elastoplastic finite element analysis, temperature field and stress field numerical simulations were calculated by ANSYS FEM software in variable polarity plasma arc welding of aluminum alloys. Combining heat source model which is Gaussian plus double ellipsoid was built. Temperature field and stress field were achieved accurately by loading different-size heat source models in positive and reverse polarity period alternatively. The values of residual stress were measured at different points in longitudinal and transversal directions after welding. The results show that the distribution law of residual stress, which was achieved in different directions on simulation model, was the same with theory analysis and the area of high stress was obviously smaller than that in TIG welding. The difference between actual residual stress and calculating residual stress is small. It proves that the calculation of numerical simulation is of great value on theory.