Abstract: The steam generator blowdown system (APG) of CPR1000 nuclear power plant is frequently automatically isolated during normal operation. In combination with the operating background of the isolation events, the causes of these events are studied, and APG is optimized. The D link is added to the cooling water temperature controller and the control parameters are optimized; the feedforward link is added to the sewage flow control loop; the corresponding delay links are added to the downstream of sewage flowmeter and pressure switch signal; the adaptive modification is made to the operation program of the start-up stage. The actual operation of the nuclear power plant has proved that the improved system is in better operation condition, the automatic isolation events are greatly reduced, and the operation and maintenance costs are effectively lowered.