Abstract: Aiming at the problems of large loss rate and high impurity content during Leymus chinensis seeds harvesting process by all-feeding combine harvesters owing to the cleaning operation process, coupled with the physical characteristics of Leymus chinensis seeds, an air-and-screen cleaning device for Leymus chinensis seeds was designed. It was designed and optimized cleaning components, feeding devices, and receiving devices. The indoor performance test of the air-and-screen cleaning device was carried out. The mechanism of cleaning performance as a function of each factor was obtained using a single factor test. The response surface test was used to establish the relationship between each factor and the impurity rate and loss rate. Through analysis of factors interaction, the best combination of working parameters was obtained, whereby speed of the vibrating screen was 275 r/min, speed of the fan was 985 r/min, and feed rate was 0.087 kg/s. At this time, the impurity rate was 27.3%, and the loss rate was 3.3%. It was established that the air-and-screen screen cleaning device met the design requirements, which can provide a reference for research and development of Leymus chinensis and other gramineous forage seeds that are fed into the combine harvester.