There exists a two-way vibration transmission between the propulsion shafting system and the shell of an underwater vessel. A single structural dynamics model cannot accurately reflect the vibration coupling effect between the propulsion shafting and the shell. In this paper, a dynamic model of the propulsion shafting-shell coupling system of an underwater vessel was established using an analytical method. The shell was simplified as a combination of underwater conical-cylindrical shell, and the propulsion shafting was modeled as a beam structure. The bearing was simplified as a spring-damper-mass system to serve as the connecting structure between the shafting and the shell. Power flow theory was used to analyze the input power flow at the shafting end and shell end under different directions of excitation, as well as the power flow transmitted through the bearing between the shafting and the shell. In addition, the vibration transfer characteristics of the shafting-shell coupling system solved by the analytical method were verified by the finite element method. The results show that the input power flow of the shafting-shell coupling system is greater under shafting end excitation, the shafting-shell coupling effect can significantly increase the input power flow under shafting end excitation, and the rear stern bearing is the main path for transverse vibration transmission between the shafting and the shell. This research provides theoretical support for vibration reduction and noise optimization design of underwater vessels.