Abstract: HPR1000 is the third generation NPP, and its reactor and primary loop system need to fulfil higher requirements for inherent safety. For the core cooling monitoring system（CCMS） of the second generation nuclear power plant, the bottom of the reactor shall be bored to measure the water level. This design degrades the inherent safety of the reactor and shall be redesigned. This paper designs a new core cooling monitoring system, in which the water level detector inserted into the core from the pressure vessel cover directly. This design improves the accuracy of measuring the water level of key points and eliminates the boring at the bottom of the reactor, which meets the HPR1000 reactor inherent safety requirements.