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The pressure fluctuation experiment was conducted and the entire flow field of mixed-flow pump was simulated by large eddy simulation (LES) with standard Smagorinsky subgrid scale model to investigate pressure fluctuations and mixedflow pump inner flow under low flow conditions. The experimental results indicated that the multiples of pressure fluctuation at the impeller inlet was the highest and it was increased when the flow rate was decreased. the main pressure fluctuation frequencies at the impeller inlet,，impeller outlet and vanes inlet were blades passing frequency under different conditions, however, the main pressure fluctuation frequencies at the vanes outlet was changed under different conditions. The large eddy simulation results indicated that the flow conditions at impeller inlet was better and axial velocity was changed small under 0.8Qopt(Qopt, operating condition). Under 0.4Qopt, however, the flow conditions at impeller inlet became complicated, therefore, the axial velocity was changed significantly. The angle of attack of the fluid on the blades was increased and affected by tip leakage flow. The flow separation was generated at the leading edge of suction surface at t* =0.0416, when the impeller was rotated from t*=0.0416 to t*=0.1249, the flow separation was intensified and the swirling strength of the separation vortex was gradually increased, so the static pressure of the leading edge of suction surface was decreased, the adverse pressure gradient was increased which promoted the generation of backflow. When the backflow reached the leading edge, the static pressure of leading edge was gradually restored,，therefore the flow separation was an important reason that making the multiples of pressure fluctuation increased.