Abstract: A kind of separating sieve that can detach scrap plastic film and cotton stubble from earth was researched. Mathematical model for motion of separating sieve was established. Motion law of scrap plastic film and sundries on the separating sieve was analyzed. Three-dimensional model for the separating sieve was made with Pro/Engineer software. By means of motion simulation, structural parameters and motion parameters of the separating sieve were optimized. Therefore, separating rate of scrap plastic film increased effectively and its vibration decreased. Experiments indicated that vibration frequency of the separating sieve was the main factor which affects separating rate of scrap plastic film. Separating rate of the separating sieve increases and the machine operates stably when flywheel turns at a speed of 175 r/min.