Abstract: This paper presents an approach to the PLC self-test and its peripheral testing system with hardware circuit in detail, which is based on the single chip of STC12C5410AD and PC software.This system can provide the analog/digital input signals required by PLC, receive analog/digital output signals from PLC and transfer data between PLC and PC software of the testing system.It can simulate industrial site and debug on-line.The realization of the system can not only improve the efficiency of the development of PLC control system, but also provide a reference for other system development.