Abstract: By microscopic visualization experiments and theoretical derivation,the regularity of CO2 gas overlapping effect mechanism in hydrocarbon reservoirs has been studied.From the experimental results and theoretical derivation analysis,under closed formation environment the CO2 gas overlapping effect depending on density difference is weak,and the single bubble could not have spontaneous migration,while continuous bubbles need to form a certain gas column length.When the bubbles column buoyancy is greater than the Jamin effect of resistance,the overlapping effect can be formed.