<p>In order to meet the demand for high-power and high-speed hydraulic pump rotor systems in the aerospace field, this study proposes a unidirectional fluid–structure coupling analysis framework based on Pumplinx and ANSYS Workbench and explores the interaction mechanism and fluid–structure coupling mechanism between different rotor components of a centrifugal gear combination multi-stage hydraulic pump. The flow field excitation vibration characteristics and critical speed mutation mechanism of multi-stage heterogeneous hydraulic pumps were revealed, and the reliability of the simulation model was verified through experiments. The main findings include: (1) The low-pressure area is concentrated on the front end face of the impeller, the inlet of the impeller blades, and the inlet end of the gear pump, while the high-pressure area is distributed at the outlet of the impeller and the meshing tooth cavity of the gear pump; (2) Cavitation bubbles are mainly concentrated in low-pressure areas; (3) Under the combined effect of hydraulic oil additional mass effect, unsteady pressure pulsation, and instantaneous impact load caused by cavitation bubble collapse, the first six modal frequencies of the rotor system decrease, resulting in a first critical speed (6,992.6 r/min); (4) The deformation at the support of the sliding bearing is the key factor that dominates the first-order critical speed, and the coupled vibration between the impeller and guide vane is the main excitation source that causes resonance in the rotor system; (5) The experimental results show that the error between the simulated and measured resonance frequencies is less than 10%, and the critical speed value (7,000 r/min) is consistent with the simulation results. This study provides a theoretical basis for the vibration reduction design and optimization of multi-stage heterogeneous hydraulic pumps.</p>