The invention provides a local area network switch monitoring device. The device comprises a PC and a switch. The switch comprises a switch chip and a monitoring module, wherein an UART interface on the switch chip is connected to a DB9 serial port of the PC through an UART bus; the monitoring module is provided with an UART interface; the UART interface on the monitoring module is connected to the UART interface of the switch chip through the UART bus and is used for detecting safety of a data packet on an Ethernet. By using the device in the invention, problems of human judgment and manual input in a current local area network monitoring system are solved; automatic identification data packet types from the system are monitored; hardware monitoring transmits data through the UART interface and decoding is difficult so that local area network safety can be increased; a monitoring scheme of combining software and hardware is provided; problems that one switch is failed and alarm is performed in the local area network; and the monitoring accesses an unauthorized switch without permission are solved.