The invention provides an automatic control boiler blowdown system. The system conducts automatic control according to the steam quantity generated by boilers and the water quantity input into the boilers. The boilers are subjected to regular blowdown. A central diagnosis monitor automatically sets the blowdown quantity according to the ratio of the steam mass to the water mass input into the boilers. According to the automatic control boiler blowdown system, by monitoring the input water quantity and the generated steam quantity of all the boilers in real time, the dynamic ratio relation between the input water quantity and the generated steam quantity is obtained; according to the dynamic proportion relation, the blowdown quantity of the boilers is automatically calculated; and according to the blowdown quantity, the blowdown time and speed are adjusted. The automatic control boiler blowdown system automatically calculates the blowdown quantity, so that compared with the prior art, the hysteresis property caused by automatic control in the prior art is reduced, and optimal blowdown control can be achieved.