Abstract: In order to improve the calculation efficiency and reduce the analysis time, an instantaneous heat source model was built to simulate the welding residual stress in a multi-pass joint, based on the commercial software MSC. Marc. A thermal-elastic-plastic finite element method was developed, and the instantaneous heat source model was used to simulate the welding residual stress in a multi-pass T-joint of Q390 high strength steel plate with thickness of 30 mm. Meanwhile, a moving heat source model was also used as a reference to verify the effectiveness of calculation. Moreover, the hole-drilling strain gauge method was carried out to measure the welding residual stress in several typical positions. The results show that the instantaneous heat source can greatly reduce the analysis time and keep a high accuracy at the same time. Compared the simulated results, it can be found that the grid density in the longitudinal direction had an insignificant influence on the computational accuracy, but coarse mesh can reduce the analysis time further.