Aiming at the problem of low vertical sweep efficiency caused by gas overlap during the CO2 flooding in thick oil reservoir，a high-temperature and high-pressure overlap model is independently developed to study the CO2 overlap migration distance. The fluid collection is completed in each layer，and the length of the core（60，50，40 cm）is changed to carry out the gas flooding experiment. The output of the upper and lower outlets characterize the development efficiency of the upper and lower parts of the core respectively to evaluate the overlap degree. The experimental results show that the migration process of CO2 under gravity segregation can be divided into three stages：the uniform gas advancement stage（0-40 cm），the transition stage（40-50 cm）and the overlap completion stage（50-60 cm）. The ratio method and area method were established to quantitatively characterize the overlap degree. The farther gas migration distance shows the bigger difference in oil production between the upper and lower outlets，the higher increase of the overlap degree index shows the bigger decline in overall oil production. Therefore，the well spacing should be reduced as much as possible to minimize the impact of gas overlap on the development efficiency while ensuring maximum economic benefits in oilfield development.