The invention relates to a system and an application which can precisely control the content of dissolved oxygen in water. The system is composed of a dissolved oxygen sensor, a transmitter, a data acquisition module, a programmable controller, a signal outlet module, two proportional electromagnetic valves, and the like, the dissolved oxygen sensor can detect the content of the dissolved oxygen in the water, detected signals are integrated to 4 to 20mA analog signals through the transmitter, the analog signals are acquired to the programmable controller through the data acquisition module, the programmable controller calculates 4 to 20mA control signals according to the measured dissolved oxygen value and the set value, and the control signals are further transmitted onto the proportionalelectromagnetic valves to regulate the opening degree of regulating valves to control the type and the flow rate of the introduced gas. The system and the application can solve the serious lag and the unbalanced forward and reverse direction control of the controlled system through the optimization of the output range of the set forward and the reverse direction control signals and the proportionof the on-off time of the control signals, precisely control the (concentration or) content of the dissolved oxygen and control the precision to be within 5ppb.