Impact damage is the main form of rice threshing and one of the significant causes of rice injury. Based on the contact mechanics, models of the compressed displacement and the largest pressure distribution were established, as the threshing component centrally collided with a rice kernel. Due to the difference of moisture content, rice kernels plastic mechanics and brittle mechanics were different. The critical speed formula about impact damage was obtained as the rice kernel collided with the threshing element. Furthermore, the critical speed was calculated for the impact damage as to the spike tooth and the rice of zhendao10, wuxiangjing14, huaidao9, zhennuo653 and xieyou084. Besides, experimental study was performed on the test platform which proved that the experimental mean value was by and large in according with the theoretical calculation.