The invention provides an automatic control system. The system comprises a monitoring diagnosis controller and a boiler; the boiler comprises a blow-down pipe arranged at the lower end of a boiler drum; a blow-down valve is arranged on the blow-off pipe; one end of the blow-down valve is connected with a valve adjusting device; the valve adjusting device is in data connection with the monitoring diagnosis controller for transferring valve openness data to the monitoring diagnosis controller and receiving an instruction from the monitoring diagnosis controller to adjust the openness of the blow-down valve; the drum comprises a water quality analyzer; the water quality analyzer comprises a pH value testing unit for measuring the pH value of water in the drum; the water quality analyzer is in data connection with the monitoring diagnosis controller for receiving the measured pH data; boiler blow-down is termly performed; the blow-down time is kept invariable; and a central diagnosis monitor automatically sets the blow-down speed according to the measured pH value to automatically control the blow-down quantity. As the blow-down speed is automatically calculated, compared with the prior art, the system has the advantages that the lagging is reduced, and the optimal blow-down control can be realized.