The invention provides a high-efficiency water-flow power generating device. The device comprises a mounting platform, a rotating mechanism, a tower tube, a machinery space, a power generator and an impeller, wherein the mounting platform is located on water, the rotating mechanism is mounted on the mounting platform, and the tower tube, the machinery space, the power generator and the impeller are located in the water; the upper end part of the tower tube is fixedly arranged on the rotating mechanism, the machinery space is located at one end, far away from the water surface, of the tower tube, the power generator is arranged at one side of the machinery space, the impeller consists of a hub and a plurality of blades, the hub is of a hollow structure, the water displacement of the hollow structure is equal to the weight of the impeller, a central shaft of the impeller convexly extends from the bottom face of the hub, a wheel ring sleeves distal ends, far away from the center of the hub, of the blades and is of a cylindrical structure, the power generator is a permanent-magnet generator and has the diameter of 2-10 meters, and the central shaft of the impeller is directly connected with a rotating shaft of the power generator. The water-flow power generating device provided by the invention is simple in structure and easy in manufacturing, the impeller rotates rapidly without overcoming the weight of the impeller under the impact of water flow, and work obtained from rapid rotation is fully converted into power for power generating, so that the efficiency of power generating is greatly increased.