Abstract: A strange phenomenon that the leak rate of the orbicular seal joint is increased with time was discovered in the process of leak tests. To reasonably explain this phenomenon, a series of studies were carried out. First, a theoretical formula for the orbicular seal joint’s leak rate was derived. Then with the help of a stress relaxation model, the relationship between the leak rate and time was obtained. The results indicate that the leak rate on the orbicular seal joint is increased rapidly firstly, then slows down, and finally approaches a constant value. They agree with the phenomenon in the practical process of leak tests.