Knotweeds are important medicinal materials in China, which have good effect in restoring circulation, dispersing stasis and relieving cough. During the knotweeds root processing, they are hit many times omnidirectionally to separate it from the soil. In order to separate knotweeds from the soil in a mechanical way, a roots-soil separating device of knotweeds was designed, which consists of hitting roller, turning-throwing roller, grille, etc. In the experiment, it was found that finger length had influence on soil-divorcing. The range of spiral angle of turning-throwing roller finger pole which is 180～275mm, was set through velocity vector projection theorem. On the other hand, the relationship between the horizontal distance about axle centers of two rollers and the size of knotweeds was developed by fitting tool of Matlab software. The result showed that rate of soildivorcing and efficiency were 91.2% and 1.35t/h, on condition that speed of hitting roller, turningthrowing roller and horizontal distance of two roller were 500r/min, 240r/min and 230mm, respectively, and therefore the device met the manufacturing requirements. It provided reference for research of soil-divorcing of crops.