Abstract: Internal components of steam generator are complex, and the reaction of primary side under loads will influence the components above the tube sheet, such as support plate and tube bundle. The axisymmetric and 3D model of lower assembly with the different simplifications of partition plate were established to study the influence of stress distribution of support plate and tube bundle. The results obtained by axisymmetric model are correct and conservative. Displacements of tube sheet with different models are variant which influence the mechanical property of support plate and tube bundle.