The large eddy simulation method with sliding mesh technology was used to investigate unsteady flow characteristic in a double-suction centrifugal pump on different operating conditions. The unsteady flow in the impeller, especially on the blade surface was focused on. The results showed that the rotating frequency dominated the pressure fluctuations in the impeller at design and off-design conditions. The amplitude of the pressure fluctuation became higher when the flow rate was farer from the design flow rate, especially at Q/Qd=0.62 that the maximum amplitude occurred, nearly three times of design condition. At design condition, the maximum amplitude of pressure fluctuations happened on the blade leading edge, nearly 14% of average pressure, which was respectively 86% and 169% higher than the centers of the blade pressure side and suction side.