In view of the high loss rate and impurity rate of rapeseed threshing selection, a double air duct air cleaning device was designed to matching the small rapeseed combined harvester, which mainly consists of the disc sorting screen, the inclined surface collector, the cleaning cylinder and the centrifugal fan. Driving by the centrifugal force produced by the rotation of the disc screen, the first screening of rapeseed is carried out to obtain the mixture of grain and small miscellaneous. When the mixtures are collected through collector and slid into the cleaning barre, the mixtures are secondly screened by the two-way air duct airflow produced by the centrifugal fan. Based on the basic equation of fluid dynamics, the air cleaning parameters of the double duct were designed. The numerical simulation analysis of the air cleaning flow field was carried out with ANSYS, and the multi-factor orthogonal test was carried out on the self-made test bench. Combined with the designed test bench, the impurity rate, the cleaning screen speed and the centrifugal fan speed were taken as the main factors to obtain the best screening parameters through single factor test and orthogonal test, taking the cleaning rate, the loss rate, and the optimal cleaning scheme as the test parameters. Theoretical analysis and numerical simulation are basically consistent with the experimental results. The experimental results show that the cleaning rate is in the range of 95.0%-98.5% for the rapeseed with impurity rate of 15%, when the feeding amount is 0.1 kg /s and under the sieve cleaning speed of 50-80 r/min and the centrifugal fan speed of 1 700-1 900 r/min. When the impurity rate was 5%, the best performance with the cleaning rate of 98.2% was achieved under the cleaning screen speed of 60 r/min and the centrifugal fan speed of 1 800 r/min.