Abstract: This paper discusses the reasons affecting the robustness in a class of expert control systems. Simulational experiments proves that expert control has more robustness than traditional PID control to most uncertain factors, but less robustaess to the disturbances acted on the system output. The theoretical analysis indicates that the reasons affecting the robustness of this class of expert control systems come from evidence uncertainty in production rules and strategy uncertainty caused by the limitations of expert knowledge. This provides theoretical basis for adopting inaccurate inference in expert control systems.