Abstract: In this paper, the dynamic property of the rotor system of a reversible bulb hydraulic pump is analyzed by using the finite element methed. Several computation models are adopted, which are approaching to the real structur from simple to complex gradually. The influences of components on the medal frequencies and the modal shapes are considered in detail.The computational results show that the first medal frequency is much higher than the operating speed of the pump, so the rotor system is safe and reliable Under the normal working condition