Abstract: In welding process the stmcture undergoes a locally high temperature cycle.The thermal expansion of the welds and nearby areas is restricted by the constraint of surrounding metals where the temperature is rather low.The compressive plastic strains in the welds and nearby area will be caused and remained at mom temperature after welding.The residual plastic strain, also named inherent strain, is considered as a source causing the welding stresses and deformations.Welding deformation of underframe is one of main factors determining the car's quality.Residual plastic strain method is applied to the welding deformation prediction of the large structure-GM's Buick's chassis underframe beam assembly which is made of four bridges with 21 welds.Two kinds of mesh sizes are used in welding deformation analysis of the underframe.The results show that 15mm is pennitted for meshes near the welds in the analysis.The analysis method and results can be used for the instruction of practical production of others cars.