According to the fundamental of kinematic, a motion model of rice in the assembled plank-tooth and helical vanes axial flow threshing and separating device was established. Through computer simulating, the variable rule of grain tangential velocity, axial velocity, absolute velocity and time has been obtained. The results indicated that grain tangential velocity is greater than axial velocity, tangential velocity is the chief ingredient in the making of absolute velocity, through high-speed photography validating, the result indicated that the test according to those motion models agreed to the theory, and the analytic result laid a solid foundation for the development of the device.