Abstract: Door flange is the critical part of the vacuum chamber of a space environment simulator. The stress analysis and the design method of the door flange are discussed in this paper. Optimized design models of the door flange are proposed. By the use of the finite element method, different models are analyzed and evaluated, and the best structure for the door flange is chosen. Engineering applications show that the optimizing design can improve the quality of the product structure as well as the reliability and the economy of the space environment simulator.