Abstract: Winnowing machine is widely used to separate lighter particles such as chaff and small stones from grain, and leave a cleaned grain for storage and processing. However, the effect of varying direction of air steam and the influences of throwing angle on the separation of winnowing are not known. In this paper, a threedimensional dynamical model of a particle moving with an initial velocity in a horizontal air steam was developed to describe the separating of winnowing machine. Its validity is tested by comparing simulated and experimental results of separating of winnowing machine. The simulated separating was observed to agree well with experimental separating. The particle motion curve was obtained by using the method of numerical analysis. The utility of the model in simulating separating of winnowing machine under various angle α and β is demonstrated by using the threedimensional surfaces. The angle α is that between air steam velocity and the throwing initial velocity of the particle, and the β is the throwing angle of the particle. The effect of air steam on grainchaff separating in winnowing was not remarkable when the angle α varied from 0 to 180, thus the winnowing machine can work with no air moving in the atmosphere. The throwing angle β was more sensitive to results of separating of winnowing machine. The good separating results of winnowing machine were achieved with the throwing angle over the range 40°～45°．