Base on small aspect ratio modeling theory, the finite element of rotor shaft coupling with disk was established, and the vibration characteristics of marine steam turbine rotor were analyzed with different modeling methods such as oblique cutting way and disk unit. Then, the significant influences of different modeling methods on the vibration characteristics of the rotor system were studied. The additional modes of rotor bearing system generated by disk unit represented as coupling vibration mode between disk and rotor shaft. The calculation method based on disk-unit model accurately described the vibration characteristics of the rotor shaft coupling with disk. For thicker disk, the disk had less effect on the rotor coupling vibration. The oblique cutting model commonly used in engineering generated additional stiffness and caused larger error in the solving process. This kind of error gradually reduced with the decreasing of the disk thickness.