The invention discloses a multi-stage filtering device for repairing a water body, and a method for repairing a water body. The device comprises a supporting layer suspended on the surface of a waterbody and a plurality of filtering units, wherein each filtering unit comprises a cuboid shell formed by geogrid plates, the shell is filled with a filtering material, the filtering material is wrappedby geotextile, the multiple filtering units are sequentially and vertically suspended and fixed to the bottom of the supporting layer from coarse to fine and float in the water body to form multi-stage filtering, and the front-back stacking direction of the filtering units is consistent with the water flow direction. According to the invention, with the application of the method as a novel and reliable combined filtering process for deep treatment of black and odorous rivers, the effective sewage interception of the water body point source and the water body purification of the drainage basinare achieved, the treatment cost can be reduced, the construction time can be shortened, the process is simple, the ecological stability can be effectively maintained, and the wide market prospect can be achieved.