A combined waste water and gas treatment system for efficiently decarbonizing and removing nitrogen, including a water feeding pump, a carbon capture device, an intermediate water tank, and an anaerobic ammonium oxidation reactor connected in sequence through pipelines, where the carbon capture device includes an anode chamber and a cathode chamber; an anode plate is arranged in the anode chamber; a cathode plate is arranged in the cathode chamber; a gas inlet pipe is further arranged at the cathode chamber; an air compressor is connected with the gas inlet pipe; a gas outlet pipe is arranged at a top of the carbon capture device; a water outlet in the intermediate water tank is fluidly communicated with a bottom end of the anaerobic ammonium oxidation reactor through a second water inlet pipe; the gas outlet pipe is fluidly communicated with the second water inlet pipe.