The organic-inorganic composite coating with a good corrosion resistance and a comprehensive nanomechanical property was in situ fabricated using chemical conversion on the surface of ZM6 Mg alloy by silanization technology in conjunction with the addition of the nano-silica and mixed corrosion inhibitor components. The chemical structure of this composite coating was investigated by FTIR and XPS, and that its corrosion resistance and nanomechanical property were studied by using EIS and nanoindentation, respectively. The results showed that the performances of this composite coating were very closely related to its chemical micro-structures. The addition of the nano-silica was useful to improve the overall nanomechanical property. The significant improvement of the corrosion resistance of ZM6 Mg-alloy with this composite coating in a 3.5 % NaCl solution thanks to the modification of the mixed corrosion inhibitors. Moreover, EIS spectra should be carefully analyzed and interpreted, due to the non-stationarity of the Mg-alloy/chloride solution system.