The invention discloses a method and a device for strengthening the denitrification efficiency of a downward vertical flow constructed wetland through electricity generation by in situ utilization ofsewage. The method comprises the following steps: step A, the sewage enters from the top of a device continuously; step B, the sewage flows into an anode conductive filler layer, and ammonia nitrogenis oxidized into nitrate nitrogen under the action of anode denitrification bacteria; step C, the sewage flows into a non-conductive filler isolation layer; step D, the sewage flows into a cathode conductive filler layer, and nitrate nitrogen is reduced into nitrogen; and step E, finally, the sewage flows out from a drain pipe in a bottom non-conductive filler layer, and total nitrogen in the drained sewage is removed. The cathode conductive filler layer is connected with the bottom non-conductive filler layer and the non-conductive filler isolation layer, the anode conductive filler layer isconnected with the non-conductive filler isolation layer and an upper non-conductive filler layer, wetland plants are planted at the upper part of the non-conductive filler layer, and an anode collector is in short-circuited connection with a cathode collector through an internal wire. The method and the device have the advantages that the method is simple, the operation is convenient, the generated electric energy is utilized in situ, and the total nitrogen removal effect of the low carbon and high ammonia nitrogen sewage is remarkably improved.