Abstract: For the unit fallback case in which the water temperature of Auxiliary Water Supply System exceeds the temperature limit in the Operation Technical Specification due to the minimum flowrate operation in Summer, the modification of installing a cooler on the line of the circulation pump for ASG tank are proposed, and the process design, changes in instrument and control system and operation control are investigated. The results show that with the modification, the water level and temperature can be maintained within the limits of OTS, and the units can operate in a safe and economic mode. The research of this paper is with a reference value for nuclear safety improvement and outage optimization.