A simple one-step anticorrosion Portland cement (PC)-based coating with an autogenerated oxide film on the surface of Mg alloy was successfully prepared in this study. The anticorrosion of coated Mg alloys was assessed by several electrochemical methods including open circuit potential, electrochemical impedance spectroscopy, and cyclic potentiodynamic polarization. The morphology and composition of PC-based coatings were characterized via scanning electron microscopy and X-ray diffraction. The composition of the autogenerated oxide film was analyzed via X-ray photoelectron spectroscopy. Results show that the PC-based coatings can significantly enhance the corrosion resistance of AZ41 Mg alloy due to the synergetic effect of PC-based coating and autogenerated oxide film. Particularly, the quality of oxide film plays a dominant role in determining anticorrosion of Mg alloy. Furthermore, the addition of metakaolin and dolomite enhances the resistance of PC-based coating and meanwhile, MoO₄^2– ions from PC-based coating can adsorb in the oxide film, improving the quality of the oxide film.