Under the rotating stall condition the periodic generation and shedding of stall cells always induce the low frequency pressure fluctuation，which has severe influence on the safety and stability of pumps. In order to reveal the role of the rotating stall cells in the pressure fluctuations, a centrifugal pump impeller was chosen as the research object to investigate by numerical simulation. A number of reference locations were arranged in the entrance of the passages for recording the pressure fluctuation. A developed large eddy simulation named dynamic mixed nonlinear model (DMNM) was applied to the numerical simulation and the simulation results showed good agreement with the experimental results. The internal flow field and pressure fluctuation characteristics under different stall operation conditions were also obtained. The simulation results showed that the stall cells first occurred in the suction side of the blade, and then moved to the pressure side with continuous decay. When the rotating stall occurred, the amplitude of pressure fluctuation was much higher than that at unstalled point. The propagation of stall cells had significant effect on the pressure fluctuations in the impeller. The dominant frequency of pressure fluctuation at blades was rotating stall frequency. The maximum amplitude of pressure fluctuation was located at the leading edge of blade, and it decreased gradually along the flow direction. As the flow rate was further reduced, the amplitude of dominant frequency decreased but the stall frequency increased. This research can provide useful reference for the secure and stable operation of centrifugal pumps.