This paper proposes a safety monitoring system for large-scale power batteries. The system is based on a Central Control Unit and a Safety Monitoring Unit. It can automatically collect important parameters such as voltage, current, temperature, and insulation resistance during the charging and discharging process of the power batteries. The relevant data of the battery pack can be uploaded to the server through Ethernet, realizing the monitoring of the power battery parameters and real-time fault diagnosis to improve the safety of the battery system. In addition, the Central Control Unit can communicate bidirectionally with the Safety Monitoring Unit through the CAN FD bus to control the charging and discharging working status of the battery pack. The test results of this paper show that the CAN FD bus can achieve high-speed data interaction within a communication distance of 15 meters. The insulation resistance monitoring accuracy of the Safety Monitoring Unit can reach 2%, and the voltage measurement accuracy is ±5mV, while the current measurement accuracy is 1%.