Abstract: According to the practical situation of K8206 working face in Yangquan NO.3 Mine, gas control effect of "U+I ventilation" model under different working face lengths are studied by using FLUENT software. Simulation results show that gas emission quantity of mining-coal seam and adjacent seams increase with the increase of working face length. Gas concentration exceeds the limit in return airway and gas tail gateway, because of the increase of air leakage rate and air leakage scope with working face length. As the working face length increases, the percentage of the gas emission quantity from mining-coal bed increases also.