Due to the greenhouse effect, the ultimate heat sink temperature of China’s coastal nuclear power sites has shown a rising trend The nuclear power plants in five coastal sites were selected as the analytic targets in this study. Firstly, comparing with its design parameter, the actual temperature of the ultimate heat sink exceeded about 1 °C at only one site. Secondly, due to the certain design margin of the relevant systems and equipment, these plants were still safe under the normal operation and startup/shutdown condition due to the certain design margin of the relevant systems and equipment, while the temperature margin of the reactor cooling pump was only 1.0 °C. Thirdly, HTRI software was used to calculate the capability of component cooling water system to discharge the generated heat during the loss of coolant accident at the hottest day of summer. The calculated result showed these plants could also keep safe under accident conditions and the temperature margin of ESW are larger than 1.95 °C. Finally, when the ultimate heat sink temperature was slightly larger than the limiting value, the thermal power is always reduced even though the plant has been proved safe and further decrease of electric energy production was resulted from the low condenser vacuum. Therefore, three solutions were put forward, including the chilled water system, helper cooing tower and industrial heat pump, which were practical way to mitigate the thermal effect and keep the safety and economy benefit.