The research of characteristics of CO2 flooding in extremely low-permeability reservoirs are of great significance.A linear dimension immiscible CO2 displacement mathematics model considering threshold pressure gradient and characteristics of CO2 which describes extremely low permeability reservoir is established. After numerical resolution, analyze the influence of threshold pressure gradient and characteristics of CO2 on the displacement characteristics of extremely low permeability reservoir. The results show that: the existence of threshold pressure gradient make average gas saturation and gas rate increase rapidly, and then the efficiency of displacement is reduced; during CO2 flooding, the expansion and viscosity reduction of crude oil make effect displacement better. The research has theory direction for the reasonable development of low permeability reservoir.