Abstract: To realize the real-time monitoring of the operation state of the negative ion-based neutral beam injection system（NNBI）, an NNBI power supply monitoring system is designed in this paper. The system integrates various types of PLC controllers through unilateral and Modbus communication protocols. A man-machine interface is developed as a platform based on the WinCC configuration software. The monitoring system provides a convenient, friendly,and stable way for experimental operators to monitor and diagnose the state of the NNBI power system. It provides necessary guarantee for the smooth development of the NNBI experiment.