The invention discloses an ammonia gas metering device, which comprises a gas tank for holding an ammonia gas. The gas tank is connected with a gas supply pipeline for supplying the gas into the gas tank, and a spraying pipe for spraying the ammonia gas; the spraying pipe is connected with a controllable proportional valve and is used for timely or quantitively spraying the ammonia gas; and a pressure sensor for detecting an air pressure in the tank is arranged on the gas tank, and is used for sending a signal for controlling the gas supply pipeline to supplement gas into the gas tank so as tomaintain the pressure in the tank to be within a preset range when detecting that the pressure in the tank is lower than a first threshold value. The gas tank is arranged for holding a certain amountof ammonia gas in the inner part, the spray pipe sprays and supplies the ammonia gas into a purifier, the ammonia gas is supplied into the gas tank through the gas supply pipeline, a gas quantity andthe pressure in the gas tank are maintained stable, and the controllable proportional valve realizes quantitively controllable and sustainable supply of the ammonia gas through a principle within thestable pressure range, so that different concrete working conditions of an automobile can be adapted. The invention further discloses the automobile and a tail gas treatment device thereof.