The invention relates to an on-line impurity control process in leaf threading and redrying. The on-line impurity control process comprises the following steps: impurity removal in the leaf spreading and throwing procedures, impurity removal in the leaf moistening procedure, impurity removal in the leaf threshing procedure and impurity removal in the packaging procedure. Aiming at the characteristics of all the procedures from the leaf spreading to finished-product offline, by combination of the state of materials and the characteristics of impurities in all the procedures, the impurity removing modes such as roller type sieving, air-separation impurity removal, manual sieving, magnetic-body impurity removal and photoelectric impurity removal are organically combined onto a leaf threading and redrying production line, so that the impurities are removed in a pointed, high-efficiency, energy-saving and systematic manner, the waste of labor, the materials and energy are avoided, first-class, second-class and third-class impurities contained in sheet tobaccos, broken chips and tobacco stems of offline finished-products can be controlled in the index range, and further the requirement of the market is met.