Residual stresses and deformation in the butt welding of an ultrathick tube-sheet in a large scale reactor are predicted by finite element method. The effect of applied load on residual stress and deformation has been discussed. When the tube-sheet is welded without any constraint and applied load, large angular deformation is generated due to the large amount of heat input, the nonuniform temperature distribution, and shrinkage. In order to decrease the angular deformation, a heavy load is applied at both ends of tube-sheet. With the applied load increase from 5 × 104 to 45 × 104 kg, the deformation decreases but the residual stress increases. When the load is beyond 45 × 104 kg, the deformation mode is changed from angular deformation to arch deformation. An optimized load of 45 × 104 kg is determined. The zone of peak residual stress is increased as the applied load increases. Too heavy a load generates a serious constraint on deformation, which in turn leads to higher residual stress.