The largescale product has a big cylinder and the pressurebearing capacity is also high. In the sealing test, the water is filled in it to have the pressure test. But due to the over high pressure, the experiment tends to destroy the cylinder and the cover to cause the failure of the sealing and the waste of the cylinder and water. So this paper is to analyze how to add the small combined cylinder within the product cylinder. The water is filled between the small cylinder and the product cylinder and the pressure on the cover is reduced during the sealing experiment.