The invention provides a device for purifying liquid ammonia decomposition gas, and the device comprises a liquid ammonia storage tank, a vaporizing chamber, an ammonia decomposition furnace, a purifier, a cyclone separator, and a drier, wherein the vaporizing chamber comprises a liquid ammonia evaporator and a buffering storage tank; the ammonia decomposition furnace comprises a preheater and a catalytic decomposer; the liquid ammonia storage tank is connected with the liquid ammonia evaporator; the liquid ammonia evaporator is connected with the buffering storage tank, and then orderly connected with the preheater, the catalytic decomposer, the purifier, the cyclone separator and the drier; the drier comprises a water-vapor absorption layer and an organic adsorption layer; liquid ammonia is vaporized and introduced into the ammonia decomposition device to obtain hydrogen nitrogen mixed gas containing little residual ammonia and impurities such as sulfur; the mixed gas is introduced into an alkaline solution through a gas distribution pipe to absorb impurities such as sulfur and NH3 in the mixed gas; the purified gas is separated, and the separated gas contains a few liquid drops; the liquid drops [S1] return to the purifier for circular purification; the separated gas is dried by the drier to obtain high-purity hydrogen nitrogen mixed gas.