Abstract: Sliver rejection from cut tobacco is based on air separating principle in the rod forming process of cigarette maker, the rejecting efficiency varies due to the air velocity fluctuation in the secondary air separation.The air velocity automatic control system in secondary air separation was improved by adding an air velocity sensor in the air passage, and modifying the regulating mechanism, the air velocity in the passage was controlled automatically via a PLC.Thereby, the efficiency of sliver and foreign matter rejecting was increased.