A method for removing ammonia nitrogen in an aqueous solution is provided in the present invention. The method includes performing an electrolysis reaction using an electrolysis device, such that the ammonia nitrogen is converted into nitrogen gas, nitrate or nitrite. The electrolysis device includes an anode including metal nickel, nickel hydroxide or nickel oxyhydroxide, and a cathode including metal copper. The method has high selectivity of converting the ammonia nitrogen into the nitrogen gas.