Abstract: A water flow power generating device, including an installing platform positioned on the water surface, a rotating mechanism on the installing platform, and a tower, a nacelle and an impeller that are positioned in water. The upper end of the tower is fixed on the rotating mechanism; the impeller consists of a hub and a plurality of blades; a central shaft of a hub is parallel to the water surface and connected to a power generator; a wheel ring is nested on a distant end of each blade remote from center of the hub; the side wall of the wheel ring is of a hollow structure or the hub is of a hollow structure; the displacement of the hollow structure of the wheel ring or the hub equals the self weight of the impeller.