Butt-welding residual stress and deformation of 390mm 20MnMoNb ultra-thick plates for manufacturing the tube sheet of a large scale EO (Ethylene Oxide) reactor were simulated by using finite element program ABAQUS. The effect of layers on the residual stress and deformation was discussed. The welding grove was double U-shaped which were welded alternatively by flipping over the tube sheet. Moreover, in order to avoid the excessive deformation of the tube-sheet, two 4 500 kN loads were applied on both ends of the tube-sheet. The results showd that angular deformation occurred in the tubesheet.  A large tensile residual stress was generated in the weld seam, the heat affected the zone near the surfaces of the tube-sheet, and compressive residual stress was generated in the weld root. As the layers increased, the deformation increased, however the stress decreased(which wass not significant). The result proves that it is not an effective method to decrease the residual stress  by increasing the layer number for such ultra-thick plates.