Abstract: Threshing injury is one of the reasons for the poor quality of rice grain. Threshing impact is an important factor causing the rice grain injury. Based on the principle of impulse and conservation of energy law, theoretical interpretation of the collisions between single, multiple rice grains and thresher were made. The mathematical model between thresher velocities and threshing broken rate was developed. On a self-designed test equipment, the experiment of relationship between thresher velocities and threshing broken rate was conducted for paddy rice. The unknown coefficients in the mathematical model were determined based on the experiments. The correctness of the mathematical model was proved. A method for researching the threshing injury to rice grain is given.