Based on the Flügge shell theory and the Helmholtz equation, taking into ac co unt the dynamic response of brace, the vibration equation coupled by the sound f luidstructure is solved. The characteristics of vibration passing through stif f ened doublecylindrical shells which are coated with acoustic covering layers a r e studied by a test. The test data shows that the coupling e ffect is quite strong when the shells are connected by the braces which mainly p ass the vibration between the shells. The laying cases of acoustic covering laye rs have little influence on the coupling effect of the shells. Th e high transmission loss composite brace structures are designed in the main passing path of vibration wave between the shells using the impedance mismatch t he ory. The vibration isolation performance of the composite brace structures are a nalyzed nomerically and the effectiveness of thenc are compared.