Abstract: To investigate the source of stems in lamina after threshing and pneumatic separation, and to effectively separate stems from lamina and promote the purity of lamina, the relationship between lamina size and stem content in lamina was analyzed. A directional separation technology was designed based on the above relationship and validated by experiments. The results showed that: 1) Vibration screen could effectively separate threshed lamina according to size. The lamina with size bigger than 42 mm (A+) had the highest stem content and the highest undesired stem content, the stem weight proportion was 3.5 times of lamina weight proportion in the A+ class. The stem weight proportion in large lamina on the screen mesh of 25.4 mm reached 56.34%, which was the key control point for the stem content in lamina. 2) The relationship between the weights of large lamina and stems was consistent with an ExpDec1 function. The weight of large lamina with size ≥25.4 mm closely correlated to the total stem weight and long stem weight, and the undesired stems mainly came from the super large lamina (A+). 3) The directional separation technology could separate stems from the large lamina effectively, the stem content in lamina was reduced from 1.64% to 0.98%, the undesired stem content decreased from 0.31% to 0.09% with a decrease percentage of 70.97%. Therefore, the directional stem separation technology provides a precise control of stem content in threshed and redried lamina and the promotion of lamina purity.