Abstract: In order to improve cleaning effect, reduce threshing losses and impurity rate of soybean thresher, the floating speeds of soybean emerging objects which included intact soybean kernels, unfilled kernels and parted kernels, unthreshed soybean pods，hard staff, broken straw and pod shell were measured by agricultural materials floating test-bed. The floating speed of intact soybean kernel was 9.18-11.61 m/s, that of unfilled kernel and parted kernel was 6.14-7.74 m/s, unthreshed soybean pods 3.81-5.48 m/s, hard staff 2.53-4.04 m/s and broken straw and pod shell 1.95-3.75 m/s. Based on the results of floating speeds test, a kind of air-suction cleaning device and tornado miscellaneous separating and circulation device for soybean thrashed debris were developed and theirs performance were tested. The results showed that when the optimal thrashing and cleaning quality was obtained with the fan rotation rate of 526-611 r/min and vibrating frequency of screen of 276-320 Hz for the developed soybean thresher, the mean straw debris rate was 0.70% and cleaning loss rate was 0.30%-0.32%. This study provides an important conference for the optimization design of pneumatic cleaning circulatory system and improvement of the performance for soybean thresher.