CO₂ miscible displacement is essential for developing low permeability reservoirs. The theoretical basis of CO₂ development in low permeability reservoirs is to understand the influence of complex structures of porous media， physical properties of fluid， and injection parameters on the displacement effect. The mathematical model of miscible displacement was established by coupling the fluid motion equation with the convection-diffusion equation. Two kinds of core pores with different structures were developed using a random four-parameter growth method， and the CO₂ miscible displacement in complex porous media was simulated.The change rule of CO₂ concentration in different pore structures and the formation and distribution of remaining oil were compared，and the influence of viscosity difference， density difference， and injection rate on the two-phase flow behavior was analyzed.Then， the theoretical production charts for different pore structures under the influence of viscosity and the theoretical trend charts of concentration and viscosity changes under different injection rates were obtained， and the characteristics of miscible displacement at the pore scale were clarified. The results show that ① there are four types of remaining oil in the miscible displacement process affected by pore structures： remaining oil at the blind end， remaining oil in clusters， remaining oil with partition， remaining oil at the tail of rock skeleton， and oil film； ② the influence of CO₂ viscosity on the oil displacement is different in different pore structures.The remaining oil content in pores is about 78%，while that in porous media with large channels is about 56%， and the two pore structures adapt to their dominant viscosity； ③ the swept ranges change as the density of injected CO₂ increases gradually. Specifically，the distribution ranges of CO₂ in the pores decrease but increase in the large channels. The comparison of mixture densities under different viscosity ratio logs shows the mixture densities under the maximum viscosity ratio log is 1.12 times that under the minimum viscosity ratio log. The maximum concentration ratio in the porous media with large channels is about 260， and that in the porous media without large channels is 450； ④ increasing the injection rate could disrupt the original displacement pressure system，increase the driving capacity， effectively reduce the distribution of remaining oil， and then improve the oil displacement effect.