Abstract: The finite element model for welding of a large-sized thin armor steel structure was developed based on the finite element software SYSWELD. The influence of welding sequence on the distortion of the thin plate structure was investigated by using Local-Global method. Using heat source fitting tool in SYSWELD, the double ellipsoid heat source models which were suitable for the site welding process were established through comparing the weld cross-sections of calculation and experiment. The welding procedures of both T-joint and butt joint were analyzed by the thermal elastic-plastic finite element method. The local plastic strain and the joint stiffness were obtained. Furthermore, the distortion behaviors of the thin plate structure in different welding sequences were simulated. The results show that the welding distortion of the medium plate is severe, and the maximum deformation is 9.6 mm. The calculated results agree well with the experimental ones. The distortion of the thin plates is improved effectively by using the optimized welding sequence. The maximum deformation is reduced by 75%.