To obtain the law and optimize factors of airflow field on the air and screen cleaning device, using the CFD software of Fluent, the numerical calculation and orthogonal experiment were implemented by changing three parameters: the obliquity of fan, the rotational velocity of fan, and the included angel of sieve. The influence law was analyzed and the airflow field was optimized by experiments results. The optimized results showed that the obliquity of fan had a great effect on the separation. The mixture of the grain and the blighted grain could be separated efficiently when the obliquity of fan was 25°, the rotational velocity of fan was 700 r/min , and the included angel of sieve was 20°.