In order to study the internal unsteady flow characteristics in the mixedflow pump caused by flow rate conditions, the external characteristic of mixedflow pump was predicted based on the standard k-ε twoequation model by using ANSYS CFX software. The influences of flow rate on internal pressure distribution, streamlines and turbulence kinetic energy distribution were further analyzed. The time domain and frequency domain responses at different monitor positions under various flow rate conditions were compared with each other. The results showed that the trend of external characteristic curve of mixedflow pump acquired by numerical simulation or experiment test was basically the same, the deviations of head and efficiency compared with numerical prediction and experimental measurement were 3.29% and 0.27%, respectively, which indicated that the numerical calculation was equipped with high accuracy. When it was operated under part loading conditions, the high pressure gradient between blade pressure surface and blade suction surface could lead to blade tip leakage flow, which had a great impact on mainstream in the impeller. Meanwhile, affected by the rotorstator interaction, more energy loss and higher turbulent kinetic energy dissipation were appeared in the impeller outlet and flow passages of guide vanes. With the increase of flow rate, the tip leakage flow was decreased gradually, the rotorstator interaction became weaker and the flow field in the impeller tended to be steady. Under different flow rate conditions, the pressure fluctuations at different monitoring positions had obvious periodic property, the time domain curve of pressure fluctuation contained four peak values and four valley values within a spin cycle of the impeller, and also the blade passing frequency always played the dominant role. Moreover, the influence of flow rate condition on impeller inlet and guide vane middle section was the most obvious and the amplitude value of pressure pulsation coefficient under part loading condition was the most dramatically changed. With the increase of flow rate, the amplitude value of pressure fluctuation in the frequency domain was decreased. This research provided a reference for expanding the operation range of mixedflow pump and optimization design of impeller and guide vanes in mixedflow pump.