Abstract: A dynamic analysis was made on the tossing motion of agricultural materials on oscillating sieve.The limit value of motion feature (K₃) was obtained, and the mathematical model on K₃ was established and calculated. The curves of the limit value of motion feature (K₃) were expressed in the graphs with the change of the screen slop angle α and vibration direction angle ε. According to the test results , it was shown that the tested value (K₃) was close to the simulation value when K₃ was smaller and it was much lower than the simulation value when K₃ was larger. The model can be conveniently applied into the determination of the design parameters in practice.