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Double -inlet two-stage double-suction centrifugal pump is a new type of pump, which has large flow rate and high head. The interstage flow channel of a double-inlet two-stage double-suction centrifugal pump is located between the first-stage single-suction impeller and the second-stage double-suction impeller. It is divided into forward channel, bridge and reverse channel. The rotor-stator interaction between the inter-stage flow channel and the impellers may be one of the main causes of pressure fluctuation. The forward channel is twin-volute type, and the reverse channel is double spiral type with a guide vane. The bridge is three dimensional distortion connecting the forward channel and reverse channel. The three-dimensional unsteady flow field of double-inlet two-stage double-suction centrifugal pump is studied by using CFD method, and the pressure fluctuation mechanism is analyzed. In the forward channel, the static pressure distribution is closely related to twin-volute type with 180°symmetric distribution under all flow conditions. The blade passing frequency is the main frequency of pressure fluctuation, and the maximum amplitude appears near the tongue. Under low flow rate conditions, the main frequency amplitude of pressure fluctuation is significantly higher than that under design conditions, which is about 18 times as that under design conditions. In the bridge area, the main frequency of pressure fluctuation is blade passing frequency under all the conditions. Under design condition, the main frequency of pressure fluctuation of inlet of the inner surface reaches the maximum along the flow direction, while that of outer surface has the reverse rule. In the reverse channel, the main frequency of pressure fluctuation is the blade passing frequency under design conditions, whose amplitude along the flow direction is gradually increased. The fluctuation amplitude in the outlet is about 11 times as that in the inlet. Under low flow rate conditions, the low frequency component and the periodic vortex nearby the guide vane are found. The research result provides a basis for improving the hydraulic design and the operation stability of double-inlet two-stage double-suction centrifugal pump.