Abstract. This paper discusses the control system based on large PLC. System diagnosis is the key function of PLC control system, also an important indicator to measure the safety and reliability of the system. In this paper, the design ideas and methods are presented and discussed in detail about signal acquisition and diagnosis, system self-diagnosis and device fault diagnosis in autonomous PLC control system. The design method is feasible to meet the requirement of industrial control and can improve the reliability of control system dramatically.