In order to improve the reliability of the hydraulic power system and the stability of performance, the two-dimensional （2D）internal flow field of gear pump was simulated and analyzed in transient state by using the FLUENT software, and the influence of the oil compressibility, viscosity and other characteristics of the oil on internal flow field of gear pump and its output pressure and flow pulsation was researched. The simulation results show that the density, viscosity, temperature and pressure of oil inside gear pump change as environmental conditions change. The cavitation phenomenon of oil occured at the meshing point of gear was obvious. When the rotational speed is 600 r/min and the load pressure is 2.5 MPa, the eigenvalue of flow pulsation at output increases 1.2 times for non consideration. Finally, the experiment results indicate that the dynamic error of flow pulsation is under 4.2%. Therefore, it provides effective tools for research about the vibration damping and noise reduction, as well as optimal design of the gear pump.