Abstract: Residual stress after seal welding and stress by internal pressure in vacuum environment of T/R box are analyzed using finite element analysis software MSC.Marc. Optimum design method is provided afterward. Effects of shape of box, length of sides and thickness of cover plate on stress by internal pressure are investigated. Effect of stress by internal pressure on welding residual stress is analyzed as well. Reliability and the limit size of the T/R box are discussed afterward. Results shown that stress by internal pressure increases by the increasing of short length and the decreasing of thickness of cover plate. The change of long length has almost no effect on stress by internal pressure.