<p>Underwater launch is a transient vibration process under the impact load. The hydrodynamic calculation is an important part of equipment research. In this paper, firstly, combining the theory of the Euler-Bernoulli beam and the potential flow, the transfer equation of the Euler-Bernoulli beam considering the hydroelastic effect is derived, which expands the application scope of the transfer matrix. The correctness of the transfer matrix for the beam with hydroelastic behaviors is verified by comparison with the results of the finite element analysis. Then the dynamics model of the underwater launch system is established based on the multibody system transfer matrix method, and the beam with hydroelastic behaviors is employed as the external vehicle. Subsequently, the overall transfer and dynamic equation are derived, resulting in the solution of the wet natural frequency and the dynamic response under the impact load. The comparison with experimental data validates the dynamics model and simulation results and ensures its reliability in practical applications. The dynamic of the system demonstrates that the launching interval and adjacent tube effect will exert an influence on the subsequent emission. This research provides a fast and efficient method for the dynamic calculation of the underwater launch system.</p>