In order to clarify the effect of permeability on gravity segregation and related mechanisms during CO2 flooding in low-permeability reservoirs，the CO2 immiscible and miscible flooding experiments on cores with different permeability are carried out according to gas gravity segregation model，and the recovery of top layer at gas breakthrough and final recovery are used to evaluate gravity segregation level and to reveal its mechanism. The results indicate that within a small permeability variation range，the increase of permeability has little effect on the increase of the viscosity and gravity，especially in miscible flooding，the viscosity and density difference of oil and gas are small，the increase of gravity segregation is not obvious after the permeability increase；the increase of permeability also increases the moving velocity of gas front and decreases the miscibility of oil and gas in immsicible flooding test，which produces a low final recovery with the influence of gravity segregation. The variation of gravity segregation with permeability can be applied to the production of rhythmic reservoirs to avoid or use the gravity segregation，especially in immiscible flooding，CO2 flooding performance in positive rhythm reservoirs could be better under the gravity segregation.