The invention discloses an oxygen supply device for alleviating eutrophication of a water body. The oxygen supply device comprises a rotating shaft, a buoyancy supporting plate, fan blades and a bracing wire, wherein the buoyancy supporting plate is of a plate structure with a certain thickness; the middle of the front side of the plate structure is hollow; a hole is formed in the side face of the plate structure; the rotating shaft penetrates through the hole in the side face of the plate structure; the fan blades are flat plates; one end of each flat plate is connected onto the rotating shaft, and the other end of each flat plate is hung; the fan blades are positioned in a vertical space in which the hollow part of the buoyancy supporting plate is positioned; one end of the bracing wire is connected to the buoyancy supporting plate, and the other end of the bracing wire is fixed at the bottom of a river channel. According to the device disclosed by the invention, the fan blades are driven to rotate around the rotating shaft by taking water flow as driving force, and due to rotation of the fan blades, air is beaten into the river water to supplement oxygen content in the river water, so that microbes in the water maintain high activities, the self-cleaning capacity of the river is improved, and pollution in the river is reduced.