Abstract: The principles of circumference control systems in KDF2/AF2 and ZL26 filter rod makers were compared.KDF2/AF2 controls filter rod diameter via regulating air pressure,which features simple structure and is convenient in maintenance;its disadvantages include:non-linear relationship between air pressure and circumference fluctuation,narrow adjustable range,no digital diameter display and inconvenient in operation.ZL26 controls filter rod diameter via regulating air pressure and airflow through PID control.The determined value has good linear relationship with actual diameter,the error of rod diameter is automatically controlled within a certain range,it improves the system's stability.