For the linear coupling of the elastic tank and incompressible and invisid liquid, there are reduced and symmetric finite element equations of coupling system. The system matrix could be obtained by the analogy method, and then the analysis of coupling system will turn to be a problem that could be solved by the general finite element software. For example, the modal characteristics of the rocket with tanks were studied. Coupling modes caused by liquid sloshing were founded, and the result shows the modes of this kind are relevant to the gravity acceleration. In addition, the greater the gravity, the higher the frequency.