Abstract: Based on the analyses with actual working conditions of air-and-screen cleaning mechanism in combine harvester, a vibro-impact motion model of material on harmonic vibration sieve was established and then a two-dimensional Poincaré map of n-1 periodic motion was derived. With the increase of vibration frequency, the variation process of Jacobi matrix eigenvalue was calculated. According to the position of eigenvalue crossing the unit circle in the complex plane, the stability of fixed point of the map was discussed and the route from periodic motion to chaos via period-doubling bifurcation was received. The system motion states dependence on working parameters were provided and the results were numerically simulated. The theoretical basis of nonlinear motion law of material on vibration sieve which is beneficial to adjustment of cleaning mechanism structure and optimization of working parameters was perfected.