The invention relates to the technical field of sludge pump equipment of a trialing suction dredger, and discloses an underwater sludge pump system of the trialing suction dredger. The underwater sludge pump system comprises a sludge suction pump, a trialing suction pipe is fixedly arranged at sludge pump connection ends on the left side and the right side of the sludge suction pump through flanges, a trialing head is fixedly installed at the right end of the trialing suction pipe through a flange, a water supplementing pump is fixedly installed at the top of the trialing head, the left side and the right side of the water supplementing pump are correspondingly and fixedly in communication with a water suction pipe and a water supplementing pipe, and the right end of the water supplementing pipe is fixedly in communication with the inner cavity of the trialing head. According to the underwater sludge pump system of the trialing suction dredger, the sludge suction pump, the trialing suction pipe and the flanges are matched with one another, the sludge suction pump and the trialing suction pipe are fixedly connected through the flanges to form a tubular structure, the position of thesludge suction pump in the whole tubular structure can be readjusted by detaching the flanges according to the different depth degrees of the sludge area excavated by the dredger, so that damage caused by making contact with sludge of the sludge suction pump is avoided.