Abstract: In order to justify the accident progresses of the cavity and the implementation effect of the cavity injection strategy that is activated under the severe accident condition, the evolution sequence of several cavity physical property parameters of different injection water velocity under severe accident condition is analyzed, and the monitoring measures in the conventional generation 2nd NPP, the improved generation 2nd plus NPP and the HPR1000 NPP are comparatively studied; Based on the optimization of the functional design and algorithm of the temperature measurement instrumentation, water level measurement instrumentation and monitoring system, the monitoring system of the HPR1000 NPP is finally designed. The monitoring system can conduct the accident status monitoring before the failure of the pressure vessel under the severe accident condition, the operation condition monitoring after the activation of the cavity injection strategy, and the molten debris status monitoring after the failure of the pressure vessel, and can satisfy the demand on the cavity status monitoring requirement under the severe accident condition.