<p>This study describes a large eddy simulation performed in a centrifugal pump impeller under conditions of sinusoidal flow rate. Five different rotational speeds, with ratios of 0.62, 0.81, 1.00, 1.19 and 1.38 relative to the design rotational speed, are selected to investigate the effects of rotational speed on alternating stall and pressure fluctuations. The research results indicate that the alternating stall phenomenon is more significant with the increase of rotational speed, especially during rising stage. The high value regions and average amplitudes of normal vorticity increase as the rotational speed increases, and the high value regions and average amplitudes in the stall passage are larger than those in the non-stall passage. In the impeller inlet and impeller passage, the pressure fluctuations in the stall passage are observed to be more complex than those in the non-stall passage. The high-frequency components of pressure fluctuations primarily manifest during rising stage and increase as the rotational speed increases. The dominant frequency of pressure fluctuations is insensitive to variations in rotational speed and position, and it corresponds to the flow rate frequency. The dominant frequency amplitudes are negatively correlated with rotational speed and decrease as the radial position increases in the impeller passage.</p>