Abstract: Stress concentration is one of the fields that attract major concerns in the structure integrity analysis of the reactor pressure vessel（RPV） of the High Temperature Gas-cooled Reactor（HTGR）. The concentrated stresses depend on the opening shape. In this paper, the boundary element method（BEM） is used for shape optimization of the HTR-PM RPV large opening. The BEM requires only boundary meshing and thus only boundary meshes move during the opening shape optimization process, avoiding the issue of volume mesh distortion. By using the shape optimization algorithm based on adaptive growth, stresses can be effectively reduced with normal growth happens in the areas of stress concentrations. Numerical tests show that optimized opening shape with low stresses can be readily obtained using the presented method.