In order to study the influence of tongue-impeller gap on the unsteady flow in a double-suction centrifugal pump, a research was presented on the internal unsteady flow, especially on the pressure pulsations. The large eddy simulation method with sliding mesh technology was used to investigate this kind of unsteady flow characteristics in a double-suction centrifugal pump at design condition. Five different impellers with different outlet diameters, corresponding to different tongue-impeller gaps were considered. The results indicated that the head rise showed an increasing tendency with lager tongue-impeller gap, while the efficiency had an overall downtrend. The results also showed that the blade passing frequency dominated the pressure fluctuations in the volute, while rotating frequency dominated in the impeller. Meanwhile, the tongue-impeller gap increasing resulted in a significant reduction of the maximum pressure fluctuation amplitude both in the volute and impeller. When the gap increased by 4%, the maximum amplitude at one time of blade passing frequency near the volute tongue was reduced by about 9%, while the maximum amplitude at two times of rotating frequency in the center of blade pressure side increased by about 38%.