Abstract: Flooding and drainage process serves as a key follow-up recovery technology in the mid-to-late stages of steam huff-and-puff in heavy oil reservoirs. The introduction of CO₂ into heavy oil development has the potential to mitigate limitations such as restricted steam chamber expansion and suboptimal thermal recovery performance; however, the coupled mechanisms governing flooding and drainage recovery remain poorly understood. To address these issues, comparative studies on steam flooding and drainage and CO₂-assisted steam flooding and drainage were conducted to study the enhanced recovery mechanisms of CO₂-assisted steam flooding and drainage in heavy oil reservoirs. A high-temperature and high-pressure three-dimensional experimental apparatus was employed to systematically analyze steam chamber evolution, remaining oil distribution, and production dynamics under the influence of CO₂. Subsequently, the experimental results were deeply fitted to establish numerical models with identical dimensions and production durations, and a flooding and drainage index was introduced to investigate the effects of CO₂ injection on the evolution patterns of flooding and drainage behaviors throughout the recovery process. The results indicate that CO₂-assisted steam enhances the well configuration advantage of flooding and drainage, increasing the recovery factor from 40.09% under steam flooding and drainage to 60.07%. After the introduction of CO₂, the thermal conductivity in the affected region decreases, resulting in reduced heat loss and an increased steam quality along the flow path. Consequently, the high-temperature zones at the top, middle, and bottom of the reservoir expand by 118.96%, 112.57%, and 236.70%, respectively, leading to an enlarged steam sweep area and enhanced oil flow capacity due to viscosity reduction. In addition, compared with pure steam flooding and drainage, CO₂-assisted steam flooding and drainage technology influences the variation patterns of flooding and drainage behaviors during the recovery process. Because CO₂ accelerates early thermal communication, reduces oil drainage resistance, and promotes the lateral expansion of the steam chamber, the CO₂-assisted steam flooding and drainage technology exhibits significantly enhanced drainage behavior during the early development stage and improves displacement performance during the lateral expansion stage. The study shows that CO₂-assisted steam is an effective approach to enhance the performance of flooding and drainage, providing important theoretical support for the efficient and low-carbon development of heavy oil reservoirs.