The structure and working principles of separating unit which is part of corn harvester were elaborated. A multi-index orthogonal experiment of 4 factors that are the type of separating roll and its velocity as well as velocity of suppressor and the distance between suppressor and separating roll was carried out, and every factor had 3 levels. In order to optimize the best operational parameters combination, the effect that parameters of the separating unit impacted on its index has been analyzed. The experimental result indicated that the type of separating roll has remarkable effect to the index of the separating unit. According to the analysis, the best plan is: the type of separating roll is type a, its velocity is 1.90 m/s, the velocity of suppressor is 2.5 m/s, the distance between suppressor and separating roll is 30 mm. Some ameliorative suggestions to the type of the separating roll were provided.