Abstract. Taking overlap joint as the object, it simulates welding process. And it obtains residual stress and distribution curves. By simulating the equivalent form of tack welding, it discusses the influence of tack welding by XYZ displacement constraint, X displacement constraint, node matching and adhesive contact on stress distribution. It verifies the results. The results show that four sets of tack welding are designed, and it calculates stress distributions. By XYZ displacement constraint, X displacement constraint and node matching, the simulation results is not consistent with the classical longitudinal stress distribution. By adhesive contact, the stress distribution is consistent with the test results, and error is controlled within 12.9%. It verifies the equivalent form of tack welding. Without considering tack welding, error is 40.3% at the distance from arc building of 50 mm. It does not meet the application requirements. It suggests that tack welding influence on the welding residual stress should be considered in the small model, and it simulates tack welding by adhesive contact.