NOVELTY - An ecological self-purifying net cage comprises a frame, net, snap rings, net ropes and multilayer biological habitat system. Upper ends of net rope are uniformly distributed around four sides of the frame, and lower ends are uniformly distributed around four sides of a sink-float plant cultivating device at a bottom of multilayer biological habitat system. Since the flat table of sink-float plant cultivating device is rigid, the net cannot retract sharply or cause damage in spite of speedy water flow.    USE - An ecological self-purifying net cage.    DETAILED DESCRIPTION - An ecological self-purifying net cage comprises a frame, net, snap rings, net ropes and multilayer biological habitat system. Upper ends of net rope are uniformly distributed around four sides of the frame, and lower ends are uniformly distributed around four sides of a sink-float plant cultivating device at a bottom of multilayer biological habitat system. The upper and lower ends of each net rope are connected on a buckle point of a flat table on the sink-float plant cultivating device of the frame. The net is fastened on net ropes via the snap rings, and its upper end is fastened on the fame. The net is rounded across the bottom of multilayer biological habitat system and approached to the buckle point of flat table on sink-float plant cultivating device at the bottom of multilayer biological habitat system. A force loaded by the net is transmitted to the frame and flat table of sink-float plant cultivating device. Since the flat table of sink-float plant cultivating device is rigid, the net cannot retract sharply or cause damage in spite of speedy water flow. Aquatic plants are cultivated in sink-float plant cultivating device and floating self-stabilizing plant cultivating container provided by multilayer biological habitat system. The multilayer biological habitat system further provides a carrier for producing an ecological film. Organisms in the net cage form an ecological environment to digest excess feed and other nutrients, to prevent eutrophication in water and to purify water.