Abstract: In order to realize real-time and overall monitoring of local running environment in electrical distributing room,a local environment monitoring system based on PLC and Modbus is designed.PLC is used to process digital I/O data,and Modbus protocol is used in data communication in this system.Various kinds of local environmental parameters are monitored and alarmed,video and audio are collected and stored.Moreover,communication with upper monitoring center by Ethernet is established.First,design scheme of total structure and function is presented,then,hardware structure and software design thoughts are described in this paper.Test results indicate that the monitoring system is stable,reliable and practical.