<p>During the test process of the high-temperature hydrogen test device, hydrogen leakage occurs at the connection of the upper and lower flange end covers, thereby affecting the accuracy and safety of the test. Therefore, the failure behavior of rubber gasket in a high-temperature hydrogen environment was studied by carrying out high-temperature hydrogen environment test, scanning electron microscopy (SEM) test, and finite element numerical simulation. Test results show that the rubber gasket experiences a seal failure after 72 hours of service at a temperature of 100 °C and a pressure of 3 MPa. SEM was used to observe that hydrogen penetrated the rubber gasket and formed bubbles, resulting in cracks. This paper analyzes and summarizes that seal failure of the high-temperature hydrogen test device is caused by the combined action of mechanical stress and accelerated aging caused by high temperature and hydrogen erosion. Three different gaskets are simulated by finite element simulation. A suggested solution is to change the cross-sectional shape of the gasket to solve the problem of hydrogen leakage.</p>