Abstract: The conical threshing cylinder that used in plot wheat breeding seed harvester has small volume and lower power consumption，it also can speed up the axial conveying of materials and improve efficiency of machine cleaning. In order to study the performance of conical threshing cylinder deeply, the comparison tests of different constructive parameters based on longitudinal axial conical cylinder threshing unit were carried out. The result indicated that cylinder taper angle and threshing components were main constructive parameters which could affect the performance of threshing unit. Then a kind of composite tooth conical cylinder with 13°cylinder taper angle and short-rasp-bar tooth was developed after optimal parameters were confirmed. The experimental results showed that while the machine was working，threshing loss rate was 0.43%, rate of trash content was 6.23%, remaining rate of seed was 0, power consumption of threshing unit was 2.48 kW, which could meet the demand of plot wheat seed harvester.