In order to optimize parameters of threshing system of corn thresher with different speeds threshing parts, and minimize mechanical damage of corn seeds in the course of threshing, an experiment of second order regression orthogonal rotation design was carried out with rotating speed of straight roller, feeding quantity and kernel water contents as influence factors and broken rate of kernel as major criterion. The results of testing showed that relation of water content and kernel broken rate was quadratic function, and the broken rate of kernel was increased with the increasing or reducing of the water contents. The broken rate of kernel was increased with the increasing of feeding quantity. When the feeding quantity was less than 0.9 kg/s, the broken rate of kernel was stable and unaffected by the feeding quantity; the broken rate of kernel was also increased with the increasing of the rotating speed of straight roller. There was significantly interactive effect for broken rate of kernel among the three influence factors. The value of kernel broken rate reached minimum with kernel water contents 18%, feeding quantity 0.9 kg/s and the rotating speed of straight roller 200 r/min. This research has important meanings for future research on mechanism of seed corn thresher and parameters optimization of threshing system, and it also is useful for developing threshing technology with low damage.