The Equipment Cooling Water System (RRI) is primarily used to cool the heat exchangers of various auxiliary nuclear island systems and transfer the thermal load to the ultimate heat sink, seawater, via the Service Water System (SEC). It also forms a barrier between the nuclear island heat exchangers and seawater, preventing the uncontrolled release of radioactivity into the seawater. During major maintenance of nuclear units, there are numerous maintenance tasks for the RRI/SEC cold source, with complex maintenance procedures. Sometimes, repairs is required due to unsuitable gasket spares or minor leaks during pressure testing of the plates, which can significantly impact outage activities and schedules. This paper presents a method for optimizing the maintenance window for the cold source system in domestic nuclear power plants through the study of key technologies such as the cooling capacity analysis of the RRI/SEC system under specific operating modes, implementation risk assessment and management. Practice has shown that, on the basis of ensuring the safety level of the nuclear power plant, applying this method can optimize the maintenance window for the cold source system in nuclear units, which helps to reduce costs and enhance the flexibility of outage activities..