Ammonia water is volatile, flammable, explosive and corrosive, which brings great difficulties to industrial storage and management. This paper presents an automatic control system for ammonia water tank based on Siemens S7-1200 PLC. The system is equipped with liquid level detection, ammonia concentration detection, pump units and valve actuators. The program development and human-machine interface configuration are completed by using TIA Portal software. It supports manual and automatic operation modes, which can real-timely monitor the liquid level inside the tank and ammonia concentration in the surrounding environment. The system automatically starts and stops the ammonia unloading pump, transfer pump and spray valve, and triggers an alarm when the ammonia concentration exceeds the threshold. Simulation results show that the proposed system has reliable control logic and stable operation. It can effectively reduce the loss of volatile ammonia water and improve the safety of on-site operation.