Recently, increasing research interest has been initiated to investigate graphene (GN)- or graphene oxide (GO)-reinforced epoxy coatings for corrosion protection. However, hardly any study has been devoted to studying their anticorrosion mechanism at a molecular level. In this work, molecular dynamics simulations were first conducted to investigate the structural properties of GN and GO nanoflake-reinforced epoxy coatings under different temperatures. The results show that the introduced GN or GO flakes can effectively improve the compactness of the epoxy coatings, while high temperature will enhance the porosity of the composite coatings. Furthermore, compared with GN-reinforced coating, more compacted coating is obtained for the GO-reinforced epoxy coating because of the stronger interfacial binding forces originating from the hydrogen bonds and van der Waals and electrostatic interactions between the polar groups in the GO flake and epoxy molecules. As a typical corrosion medium, dynamic diffusion of guest water in the composite coatings was also simulated to estimate the effects of the added GO or GN flakes on corrosion protection. The results show that the added GN or GO flakes can act as a physical barrier for water diffusion, while the GO flake can further adsorb the guest water and restrain its movement. Finally, a jump-diffusion model of water in epoxy coating is unraveled. The results obtained in this work deepen our understanding of the anticorrosion mechanism of GN- and GO-reinforced epoxy coatings.