The invention relates to a device and method for eliminating the cathode passive film of a magnesium electrolytic cell. The device comprises a dewatering bin. A feed pipe is communicated with the upper portion of one end, in the transverse direction, of the dewatering bin, and a discharge pipe is communicated with the lower portion of the other end, in the transverse direction, of the dewatering bin. The discharge pipe is in a bent L shape. The vertical part of the L shape is the vertical section of the discharge pipe, and the horizontal part of the L shape is the transverse section of the discharge pipe. The feed pipe is communicated with a gas inlet pipe. The gas inlet pipe is provided with a gas inlet valve. The feed pipe is provided with a feed valve. In the use process, the discharge pipe of the device extends into the electrolytic cell which is running, the opening of the discharge pipe extends into a cathode, magnesium chloride hexahydrate is put into the dewatering bin of the device and is dewatered through heat of electrolyte so that magnesium chloride dihydrate and magnesium chloride monohydrate can be formed, inert gas is introduced from the gas inlet pipe to blow hydrate to the cathode of the electrolytic cell to react so that the cathode passive film can be eliminated, and existing current efficiency of the electrolytic cell is recovered.