Abstract: A special-shaped vacuum vessel (with square structured lower part and semi-cylindrical structured upper part) has been analyzed and improved by finite element method.The results show that the human door should be opened in the place where the deformation of the cylinder is smaller or consistent to prevent the influence of cylinder deformation on the flange seal of human door.As the investigated vessel is connected to another vessel through the bellow,the stiffening ring with a fixed constraint at the bottom will cause a significant distortion.It makes the cylinder offset about 11 mm,and the stress intensity will exceed the allowable value.Besides,the design of the "Pi" type strengthening structure is effective in improving this situation,and the vertical deformation of the end face is reduced by 44.78% after the structure is strengthened.