Abstract: Introduced is a rule-based auto-tuning of PID controllers. According to the expanded critical proportion band method and expert system idea, items of practical experience to tune PID controllers are given in production rules, By both building the rule base of PID contoller setting and employing forward-chain inference, the auto-tuning of PID controllers has satisfactorily been completed. Relevant knowledge base and inperene engine are discuss, velerant knowledge base and inference and expermental results are shown.