In this research, to evaluate the welding distortion of stiffened thin-plate structures, an analysis method was developed that considers large deformation theory in elastic analysis using inherent strain based on Idealized Explicit FEM with shell elements. The developed method was applied to the analysis of stiffened thin-plate structures. The following factors were considered in the analysis to investigate their influence on welding distortion: heat input and welding processes such as double-sided continuous fillet welding, single-sided continuous fillet welding, double-sided intermittent fillet welding, and staggered fillet welding. From the analysis, it was clear that a buckling-type deformation is generated when the heat input is large. Regarding the welding method, the largest deformation was observed in double-sided continuous fillet welding. In addition, to show the applicability of the developed method to large-scale problems, the developed method was applied to the analysis of the car deck of a pure car carrier ship. As a result, it was found that the developed method can analyze the welding deformation of a large-scale stiffened thin-plate model in practical computing time.