A prevailing perception on Mg metal anodes is that high ion-diffusion barriers in corresponding passivated interfaces can induce poor reversibility and high overpotential. However, the dynamic evolution and degradation of native solid/electrolyte interfaces (SEI) in the electrochemical process has not yet been established. To unravel the origin of unstable Mg anodes, this study comprehensively reveals the native SEI is dominated by organic components. A model system with controllable electronically insulating SEI is designed by increasing the inorganic component, to provide a new insight that the interfacial electronic property and composition is decisive to the degradation of Mg metal anodes. The initial organic-rich SEI with insufficient electrical insulation in turn hinders ion transport by undergoing continuous cracking/reformation and electronic leakage which induces continuous proliferation during operation process. By optimizing electronic insulation of the initial interface, a symmetric cell exhibits superior cycling performances of over 1150 h with low polarization. Display omitted •The native interfacial composition plays the decisive role in the interfacial dynamic evolution.•The organic-rich SEI with insufficient electronic insulation leads to the interfacial degeneration.•A simple model is proposed to clarify the inorganic-rich SEI with electronic insulation is crucial for stabilizing Mg metal anodes.