Flat rotary screen is an important grain screening and cleaning device during grain storage and processing. The work aims to study the effect of the various screening parameters on the screening performance of rotary screen, and find out the optimal screening parameters. Based on the Discrete Element Method (DEM), numerical simulation was carried out to the rotary screening under different parameters. Four variables of screening flow, rotation speed, rotation radius and screen inclination were adopted in turn to conduct single factor analysis. Then, the effect law of each parameter on screening performance indexes of screening efficiency, impurity removal efficiency and material flow speed was obtained, and the optimal level range of each factor was selected for multi-factor orthogonal test. Finally, the test results were weighted by matrix analysis method. The better parameter combination of comprehensive performance evaluation indexes was obtained:screening flow of 3.0 t/h, rotation speed of 465 r/min, rotation radius of 24 mm, and screen inclination of 9°. Seeking better screening efficiency will inevitably reduce the impurity removal efficiency and material flow speed, that is, the longer retention time on the screening surface can increase the chance for materials to pass through the screen. Therefore, the probability of impurities passing through the screen is also increasing.