In order to solve the problems of low communication rate of traditional controller backplane,difficult expansion and huge data volume after expansion,an expandable PLC system based on Ethernet backplane bus is introduced.The overall structure of PLC system and the hardware design principle of main subsystem components are introduced from the perspective of hardware design,including central processor module,communication card and expansion communication card module,as well as the system expansion scheme.The PLC system software category and Ethernet backplane bus communication strategy are introduced from the perspective of software design.This system scheme can realize the random combination of various I/O functions,and the system capacity can be easily increased without significant changes in software and hardware.