The use of superior damping composite materials for submarine vibration and noise reduction design has been relatively common, as a typical construction unit form, the damping properties of a steel/composite plate-shell composite structure are closely related to the response of the structure in the resonant region. Based on the viscoelastic damping theory, the damping dissipation mechanism is described, and the composition of the loss factor in the system is discussed based on the statistical energy theory. It is pointed out that the structural loss of a variety of composite materials is related to the material composition and mode, and the sound radiation loss is obviously affected by the fluid medium and shell nature. The internal loss factor tests of the steel plate and the glass fiber reinforced resin shell plate are completed in four working conditions. Combining with numerical calculations, the loss law of the experimental model is summarized, and its boundary loss factor is predicted. The results show: the sound radiation loss in the air is only about 1/40 of the water’s in the same model composition and boundary conditions; the boundary loss contributes greatly to the internal loss factor of the system in the condition of screws, laps and other nonrigid form of connection, particularly for the steel plate model. The overall damping performance between this kind of connection form and free boundary state exists a difference of several times.