In this study, the waterproofing mechanism of double seal gaskets is qualitatively analyzed and the concept of breakdown water pressure and a numerical estimation formula of gasket breakdown water pressure based on elasticity are presented. The value of the breakdown water pressure is verified using a double seal gaskets test, identifying the factors affecting the waterproofing performance of double seal gaskets. The study results indicate the following: (1) When the external water pressure is greater than 1.36 times the initial waterproofing capacity of the outer gasket, the gasket breaks down completely and loses the residual waterproofing capacity. (2) Different gasket arrangements will affect the waterproofing performance of the double seal gaskets during the actual application. Additionally, it is recommended to arrange gaskets with stronger waterproofing capability on the outside, and the waterproofing capability difference between two gaskets should be minimum. (3) The structure and size of the seal groove affect the gasket′s waterproofing performance. (4) The segment dislocation of the seal gasket will affect the waterproofing performance of single and double seal gaskets. Therefore, segment dislocation must be avoided.