A purification method mainly used for deep dehydration and removal of residual ammonia from ammonia decomposition gas, which adopts alternating operation of two towers, with tower A working and tower B regenerating. Nitrogen can be used for regeneration of the dryer to reduce the consumption of liquid ammonia. Through this device, the residual ammonia content of ammonia decomposition can be controlled to be less than 3ppm, and the dew point of water content can be controlled to be less than -70 ℃