Fire, as a typical internal hazard considered in nuclear power plants design, may directly damage large numbers of cables and instruments and lead to loss of accident monitoring information, which can be a serious obstacle for operator executing accident operation procedure to control the unit state. In this paper, a method for identifying the monitoring information required by unit operation under a fire in nuclear power plants is proposed and the corresponding protective measures for various information are suggested. After comprehensively analyzing the accident operating procedures under fire conditions, the fire vulnerabilities are screened out and eliminated, thus correct implementation of accident operation procedures is ensured. The method has been successfully applied to fire safety design of HPR1000 nuclear power plants and is expected to be more widely used in other nuclear power plants.