Aiming at the limited spatial layout of hilly flax combined harvester and the slight difference in flax exfoliate traits after harvesting, the large degree of mixing, and difficulty in the clearing, to improve the cleaning efficiency of hilly mountain flax combined harvester, and explore the working mechanism of hilly flax integrated harvester compound sorting system, the primary selection mode of hilly flax combined harvester+precise compound cleaning system was taken as the research object and the preliminary selection system, the selection system CFD model and the flax extract DEM model were established respectively. CFD-DEM combined simulation technology was used to study the optimal working parameters of the hilly flax integrated harvester compound sorting system and the movement trajectory and spatial morphology of each component of the extractant. The separation law of the extract of hilly area flax combined harvester was obtained, and the verification test was carried out to verify the reliability of the simulation model. The CFD-DEM co-simulation results showed that when the inlet wind speed of the primary cleaning system was 12.4m/s, the seed loss rate of the preceding cleaning system was 0.3%, the short stem discharge rate was 71.43%, the glumes discharge rate was 69.34%, the light impurity discharge rate was 65.34%, and the impurity content rate was 39.01%, and the centrifugal fan speed of the precise cleaning system was 1.154r/min, the seed loss rate of the precise cleaning system was 0, the short stem discharge rate was 40%, the glumes discharge rate was 75%, the light impurity discharge rate was 100%, and the impurity content rate was 2.56%. In the primary system, the initial instantaneous velocity and displacement of the explants entering the airfield change in the order of light impurity, glumes, flax seeds, and short stalks. In the precise system, the initial instantaneous velocity and displacement of the explants entering the airfield were in the order of glume shell, light impurity, short stem, and flax seed. The results showed that when the water content of flax seed was 5.34%, the impurity rate of the machine was 3.61%, and the total loss rate was 1.98%. The whole machine ran smoothly during the operation, and the operation indexes met the standard of mechanized flax harvest. The experimental results agreed well with the simulation results, which verified the model's reliability. The research result can provide some reference for the design and working mechanism research of the cleaning system of flax combine harvester in the hilly area.