The migration law of CO2-crude oil miscible zone has an important influence on the effect of CO2 miscible flooding，but the influence has not been systematically understood yet. Through the compositional numerical simulation and mathematical derivation，this paper studies the migration law of the miscible zone and its influence on the recovery factor.The results show that the dimensionless area and the sweep efficiencies at the leading and trailing edges of the miscible zone increase linearly before the leading edge of the miscible zone is broken through. At the same time，the dimensionless width gradually decreases after reaching a peak value of 0.14 and the fingering coefficients at the leading and trailing edges grow faster and faster；the miscible area reached a maximum of 0.22 when the leading edge of the miscible zone is broken through. The rising speed of the sweep coefficient at the leading and trailing edges drop significantly after the trailing edge of the miscible zone is broken zone through. Meanwhile，the dimensionless area，dimensionless width，and fingering coefficients go down gradually and then stabilize. The oil displacement efficiency and expansion coefficient remain approximately constant before the breakthrough of the leading edge and after the breakthrough of the trailing edge，which means that the increase in the sweep efficiencies of the miscible zone is the primary factor affecting the development effect of CO2 miscible flooding. For this purpose，before the miscible zone is broken through，measures such as soaking，water-alternating-gas injection，injection-production coupling，and plugging of the main flow line should be adopted to expand the sweep efficiency.