To avoid uneven vertical displacement of CO2 flooding in beach-bar sandstone reservoirs with extra-low permeability，we analyze the influence of reservoir permeability，crude oil viscosity，oil saturation，and oil layer thickness on layer combination by the numerical simulation. The orthogonal test method is used to determine their degrees of influence in descending order：crude oil viscosity，oil saturation，reservoir permeability，and oil layer thickness. Through formula derivation and numerical simulation of reservoirs，comprehensive effective fluidity that involves oil saturation，crude oil viscosity，and reservoir permeability，as well as starting pressure gradient and fracturing，is proposed to characterize layer combination. The limits of comprehensive effective fluidity under various injection-production pressure differences are determined with the numerical simulation technology for reservoirs. As the injection-production pressure difference increases，the limit gradually decreases. According to the comprehensive effective fluidity of each oil layer，the K-means clustering algorithm is used for the automatic division of layer combination. The research results are applied to the Es4 in Block Gao89 of Zhenglizhuang Oilfield，showing that the recovery is raised by 2.25% when the layers are divided into three schemes.