In this paper, numerical simulation was carried out for the tube bundle of a slurry oil steam generator with concentration on the thermal stresses at the tubesheet with or without a welding clad on the tubesheet surface. It is found that as having a larger heat expansion coefficient, thermal expansion of the welding clad is constrained and most areas are in compressive state. But the tensile stresses in the clad are also notable especially at the interface and could break the clad if added by the tensile stresses produced by pressure loadings. Presence of the welding clad causes significant tensile stresses in the base tubesheet. It is possible that the maximum tensile stress comprised by the thermal tensile stress and pressure induced tensile stress will exceed the tensile strength of the material and cause initiation of cracks in the tubesheet.