To determine the law of stress on the plastic shells of image intensifiers and to identify the characteristics of quality change, the deformation theory is used in this study to establish a mathematical model of silicone rubber deformation and to fit the stress curves of plastic shells. Accordingly, the inevitability and life-cycle characteristics of the plastic shells of image intensifiers are determined. The results show that the mathematical model is in good agreement with the experimental data and can characterize the stress conditions and life variations of plastic shells.