Abstract: The vacuum vessel is the main part of a space environment simulator, to be used for the spacecraft-level vacuum thermal test. In view of safety and reliability, the loads (such as the external pressure and the deadweight) must be accurately calculated when the vacuum vessel is designed. The stability analysis based on the finite element method is carried out in order to acquire the stress and the critical instability pressure. By the estimation and the analysis for the stress intensity and the stability, reliable results can be obtained for the vacuum vessel’s design and manufacture.