The CO2-crude oil miscible zone has an important influence on the development effect of CO2 flooding. At present，no reasonable miscible zone characterization method has been proposed. Through the theoretical analysis and component numerical simulation，the formation mechanism and characterization method of CO2-crude oil miscible zone are systematically studied. The results show that the formation of the miscible zone is dominated by the vaporization-condensation between CO2 and crude oil，and the evolution after the formation of the miscible zone is mainly affected by the hydrodynamic dispersion. The isoline with the oil-gas interfacial tension equal to 0.1 dyn/cm is taken as the trailing edge of the miscible zone，and the isoline with the CO2 content in the oil phase equal to 0.2 is taken as the leading edge of the miscible zone.The width of the miscible zone is equal to the ratio of the area of the dimensionless miscible zone to the length of the envelope of the leading edge of the miscible zone. As the cumulative injection volume increases，the width of the miscible zone gradually increases but the growth rate gradually decreases before the leading edge of the miscible zone breaks. After the leading edge of the miscible zone breaks through and before the trailing edge of the miscible zone breaks，the width of the miscible zone decreases rapidly and then stabilizes. Based on the accurate characterization of the miscible zone，it can provide a basis for the regulation of the miscible zone to improve the development effect of CO2 flooding.