The invention discloses a full-automatic ammonia water preparation system. The full-automatic ammonia water preparation system comprises a PLC touch screen control system, an ammonia distribution box,a first one-way valve, a second one-way valve, an aeration pipe and a plurality of automatic valves, the PLC touch screen control system is correspondingly and electrically connected with a first online conductivity meter, a second online conductivity meter, a first pressure-type liquid level transmitter and a second pressure-type liquid level transmitter, the aeration pipe is installed in the ammonia distribution box, and the multiple automatic valves are installed on a pipeline between the aeration pipe and an ammonia gas source. According to the full-automatic ammonia water preparation system, through the relationship between the ammonia water concentration and the ammonia water conductivity, the ammonia water concentration required to be prepared is judged according to the value of the ammonia water conductivity; through the logic control of each automatic valve by the PLC, the full-automatic preparation of the ammonia water is achieved, human monitoring is not needed, safety is achieved, and worry is saved; and a raw material for ammonia water preparation is changed from the purchased concentrated ammonia water to liquid ammonia, and the ammonia dosing cost of an ammonia dosing system is greatly saved.