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The three-dimensional, unsteady Reynolds-averaged Navier-Stokes equations with shear stress transport turbulent (SST) models were solved to investigate the flow field and the characteristic of pressure fluctuations in the double volute. The results show that the pressure fluctuations are strong in the double volute. The pressure fluctuations in the volute are relatively low at the design flow rate condition. The blade passing frequency dominates the pressure fluctuations near the volute tongue and the clapboard at the design flow rate condition. The amplitude of the pressure fluctuation is largest at the beginning of clapboard, which is 25 times larger than at the volute tongue. At the large flow rate condition, the blade passing frequency is also dominative in the pressure fluctuations, but at the small flow rate condition the rotation frequency becomes dominative. The radial force on the impeller is unsteady especially at the small flow rate. At three flow rates, the radial forces all point at the beginning of clapboard.