The study on the oxidation process of magnesium (Mg) alloy and corresponding control mechanism by in situ real-time observation has been a hotspot in the field of surface treatment for magnesium. In this study, magnesium film was kept for 10 h at 400 ℃ in a furnace, and the faceted Mg nanopores were prepared by focused electron beam technology under a transmission electron microscope (TEM). The slow oxidation and growth dynamics of the surface lattices at the vicinity of Mg nanopores under trace oxygen condition were observed using in situ high-resolution transmission electron microscopy techniques, while the corresponding mechanisms were also researched. The results show that the oxidation proceeds via an adatom process, resulting in the diffusion of Mg atoms from the substrate to the subsurface of oxides, and the oxidation growth process is mainly achieved via layer-by-layer epitaxial growth of MgO. The faceted morphology enclosed by {200} lattice planes attributes to the typical anisotropic growth. The study on the roles of defects in the process of oxidation growth shows that the defects, such as vacancies and dislocatios, promote the oxidation process, whereas the large-angle grain boundary of the twinning inhibits the rotation or migration of the grain boundary, which is conducive to improve the corrosion resistance of magnesium alloy.