The invention discloses an automatic ammonia-water preparing device and method. The automatic ammonia-water preparing device comprises an automatic ammonia-water preparation control cabinet, a drug dispensing box and anti-ammonia-slip device, and an ammonia-gas detecting and warning system, wherein the automatic ammonia-water preparation control cabinet and the drug dispensing box and anti-ammonia-slip device are connected; the conductance ratio of ammonia water in a drug box is detected by utilizing a conductivity meter connected with a conductivity electrode in the drug box, and the concentration of ammonia water is obtained by the matrixing of PLC in the automatic ammonia-water preparation control cabinet, when the concentration of ammonia water is lower than a set value, the PLC sends out a command to open an electric ammonia-gas inlet valve, and when the concentration of the ammonia water in the drug dispensing box reaches the set value, the PLC sends out a command to close the electric ammonia-gas inlet valve; when a liquid level in the drug dispensing box is lower than the low threshold of the set value, the PLC sends out a command to start an electric desalting water inlet valve, and when the liquid level reaches the high threshold of the set value, the PLC sends out a command to close the electric desalting water inlet valve. According to the automatic ammonia-water preparing device disclosed by the invention, manual operation is not needed during the preparation of ammonia water, the concentration of the prepared ammonia water and the liquid level of the drug dispensing box are set by the PLC.