When simulating and analyzing the newly designed dredge pump HDP-263-50-130-4 with sliding meshes, we find that the pump head, shaft torque and hydraulic efficiency are all in periodic fluctuation although with same rotate speed and flowrate. The fluctuation could be approximated using a sinusoidal function whose amplitude is relatively large, and its period equals the time in which the impeller rotates through the angle of two adjacent blades. The fluctuation of pump head keeps a step of about 0.21 fluctuation period ahead torque, while the efficiency curve is ahead of torque about 0.34 periods. When a blade turns to section I of the volute with a 29° angle between them, the pump has a minimum efficiency of about 77%. And when this blade passes section I of the volute and makes an angle of about 16° with section, the pump presents a maximum efficiency of 92%. We draw the conclusion that the fluctuation of pump’s hydraulic performances should be related to the interaction between the rotating impeller and static volute. And the interaction could be mainly presented by the variation of flow field around volute tongue due to different locations of blades relative to the tongue.