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Flax is one of the important oil crops in Northwest China, and Gansu Province is one of the main producing areas of flax. To enhance the mechanization level of separating and cleaning of flax threshing materials, a double duct system of air-screen separating and cleaning machine was designed for flax threshing material. For further study of the improved working performance of double duct system of air-screen separating and cleaning machine for flax threshing material, the single factor parameters obtained with the analysis were determined by practical test method and discrete element simulation, the Box-Behnken experimental design with three factors and three levels was performed, parameters such as vibration frequency of screen box, air volume adjustment gear in front duct and air volume adjustment gear in rear duct were selected as independent variables, which could influence the seed impurity rate and the cleaning loss rate. The mathematical model was established, and then the effects of various parameters and their interactions were analyzed as well. The test results showed that effects orders of three parameters on the seed impurity rate and the cleaning loss rate were as follows: vibration frequency of screen box, air volume adjustment gear in front duct and air volume adjustment gear in rear duct. The optimal working parameters were as follows: the vibration frequency of screen box was 2Hz, the front duct air volume control handle in 2 gear and the rear duct air volume control handle in 4.5 gear. Confirmatory tests showed that the average value of the seed impurity rate and the cleaning loss rate were 0.98% and 2.66%, which were dropped significantly compared with that before the optimization, the degree of impurity and loss of flax threshing material in mechanized separation and cleaning could be reduced under the condition of optimizing working parameters, and compared with the single air separation method, the operation effect was significantly promoted.