The invention relates to a negative-pressure ammonia stilling system and a negative-pressure ammonia stilling process method. A raw material aqua-ammonia pump is connected with the upper part of a negative-pressure ammonia stilling tower through a mixing pipeline; an ammonia dephlegmator is arranged at the top end of the negative-pressure ammonia stilling tower; the ammonia dephlegmator is connected with an ammonia condenser; the lower part of the ammonia condenser is connected with a condensate tank; the condensate tank is connected with a first pipeline through a condensate pump; one side of the ammonia condenser is connected with a vacuum pump; the vacuum pump is connected with a coal gas pipeline; an outlet in the bottom of the negative-pressure ammonia stilling tower is connected with one side of the lower part of a heat exchanger through an ammonia stilling wastewater pump; a circulation ammonia tank is connected with the bottom of the heat exchanger; the upper part of the heat exchanger is directly connected with one side of the negative-pressure ammonia stilling tower; an alkali tank is connected with the mixing pipeline through a metering alkali pump; a cooling water supply tube is respectively connected with the ammonia dephlegmator and the ammonia condenser; a cooling water return tube is respectively connected with the ammonia dephlegmator and the ammonia condenser. The negative-pressure ammonia stilling system which is simple in structure and capable of effectively reducing energy consumption and stably stilling ammonia is provided.