80mm。 Rice kernel damage may become worse during threshing, while performance indexes of threshing unit such as un-threshed loss rate, power consumed, et al. were most focused on. Broken rate of threshing and stand damage index increment were used to validate damage degree. On the self-designed test equipment of the threshing separation performance, experiments were conducted for rice with different factors such as cylinder-concave clearance，linear velocity of spike tooth, feed rate and distance of spike tooth. With the orthogonal experiments, the analysis of variance and analysis of range were carried out and the factors that affecting rice damage during threshing were found, they were in turn of linear velocity of spike tooth, cylinder-concave clearance，distance of spike tooth. With the method of synthetic analysis, the optimization parameters were obtained. 