The grain mixture are separated from threshing of headfeeding combine harvester，which was distributed uniformly by grain pan，but the grain mixture distribution was still non-uniform on top sieve：the both sides had many grains and the middle had less, which affected the cleaning quality. The theory of “the sieve surface grain mixture was uniform with crosswise air flow velocity” was advanced. When double conical centrifugal fan working，due to the wind pressure difference Δp between the large end and the small end of the impeller which had the K-shaped blade, can produce crosswise air flow velocity. Depending on the total pressure, air volume and wind speed of the air flow cleaning required for the feeding amount, double conical centrifugal fan with impeller which had the K-shaped blade was designed. The theoretical pressure difference Δp and the transverse wind speed were calculated；comparison analysis of value simulation for cleaning flow field was carried out and quadratics rotary orthogonal combination design experiment and the multi-objective optimization calculation was carried out. Theoretical analysis and experimental verification showed that the diameter difference between the small-end and large-end of conical centrifugal fan caused the wind pressure difference，which resulted in crosswise wind velocity in the front of vibrating screen，puff falling grain mixture to middle zone from the both sides. When the large-end impeller diameter x1=308.72mm，fan rotation speed x2=1186.71r/min, blade taper x3=3.12°， the maximum crosswise wind speed may reach 3.26m/s，cleaning loss rate y1=1.08%，impurity rate y2=0.68%, which improved the cleaning quality of the head-feeding combine harvester.