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In order to solve the problem of poor threshing ability because of the transverse space limitation and grain mound in the entrance of longitudinal cleaning sieve of axial flow threshing—separating—cleaning unit in combine harvest, a new type axial-flow differential-speed threshing—separating—cleaning unit was designed, which consisted of coaxial differential-speed threshing cylinder, conical centrifugal fan, double-layer vibration sieve and rethreshing system with helical plate toot. The test-bed at feed rate of 2kg/s for rice was developed, the optimal structural parameters and operation parameters were explored by quadratics rotary orthogonal combination design experiment．The mathematical models of loss rate, crushing rate, impurity rate and power consumption were established by means of regression analysis. The influences of rotary speed combination of differential-speed roller, blade taper of conical centrifugal fan and length ratio of differential-speed cylinder on loss rate, crushing rate, impurity rate and power consumption had been analyzed, and multi-objective optimization based on Matlab optimum toolbox was applied. The results indicated that the sequences of factors influencing the threshing and cleaning performance about loss rate and impurity rate were rotary speed combination of differential-speed roller, blade taper of conical centrifugal fan, length ratio of differential-speed cylinder, and the sequences of factors influencing crushing rate and power consumption were rotary speed combination of differential-speed roller, length ratio of differential-speed cylinder and blade taper of conical centrifugal fan. The values of optimum scheme were 750r/min and 850r/min, 3.8°, 30%, respectively. The loss rate was 1.57%, broken rate was 0.71% and impurity rate was 0.38%. The total threshing power consumption was 6.67kW/kg, and the low speed threshing cylinder and high speed threshing cylinder accounted for about 59.3% and 40.7% of total threshing power consumption, respectively. The results of field experiment indicated that the indexes of working performance of axial-flow differential-speed threshing—separating—cleaning unit were more effective than national standards. The study would provide some theoretical basis for the design of axial-flow differential-speed threshing—separating—cleaning unit.