The design framework of human-machine interface (HMI) provides guiding principles for the display of digital information on the system interface. In order to solve the problem of human error caused by the unreasonable design of information interaction interface of complex system, this paper takes the feedwater flow control system (ARE) of nuclear power plant as an example to optimize . The design of the human-machine interface of the system is based on the framework of function behavior state (FBS), defines the mapping relationship between various levels of the system interface, defines the information requirements of the monitoring system interface and presents the information according to the cognitive decision-making theory, and improves it on the basis of the original interface. The purpose of this research is to make the monitoring interface more consistent with the mental model of the operator and reduce the cognitive workload and error probability, so as to ensure the safe and reliable operation of the nuclear power plant. This research also provides some reference for the optimal design of the HMI of the monitoring system of the nuclear power plant in the future.