The longitudinal vibration of the ship's shaft system is an important factor affecting the overall safety performance of the ship. The oil film in the thrust bearing is changed when the shaft speed changes, reflecting the longitudinal vibration characteristics of the shaft system. The time-varying situation of the oil film in the thrust bearing was analyzed to investigate the relationship between the oil film and the longitudinal vibration, and the lubrication condition of the thrust bearing oil film was solved using the Reynolds equation. The oil film stiffness model was obtained by solving the finite difference method and the relaxation iteration method, and the longitudinal vibration of the shaft system as a whole was analyzed using the Lagrange equation. The influence of the oil film on different damping methods was compared. The results show that the change of the oil film stiffness mainly depends on the size of the thrust bearing in a certain speed range. The semi-active damping method is less affected by the oil film stiffness, but the influence will increase as the shaft system speed increases. In the design of the absorber parameters, there is still room for improving the damping effect by not considering the oil film change when designing the absorber parameters. For example, by increasing the damping ratio in the absorber, a better damping effect can be achieved.