Principle of coaxial differential threshing and it’s design schemer was proposed. Axial distribution curve and mathematical model of threshing mixture were established in the case of single speed threshing and differential speed threshing by means of Matlab on the basis of experiments. Results showed that grain total percentage of damage was 0.67%，the quantity of impurity with grain was 8.63% of total mixture quantity， the grain loss rate of unthreshed head and lined grain was 0.76%，which reduced by 40.9%, 29.5% and 32.15% of the single speed rotor. Differential threshing quality was better than single one. Coaxial differential threshing units integrated with the beneficial effects of high-speed threshing and slow-speed threshing, which could reduce grain loss rate, damage rate, impurity rate of three performance indicators synchronously and achieve better level.