Mixed metal oxides constitute a common kind of material in nature, in which Mg-Al mixed oxide (MMO) is widely distributed in the basic-ultrabasic magmatic rocks, evaporates of rivers-lakes and marine sedimentary rocks. In this study, MMO was obtained by calcining of Mg-Al hydrotalcite. Its rehydration features and the related transformation mechanism in silicic alkaline solution under hydrothermal condition were investigated by changing reaction time. X-ray diffraction (XRD) patterns showed that MMO reconstructed to form hydrotalcite firstly, after that hydrotalcite transformed to saponite gradually, along with the formation and disappearance of impure phase (e.g., natrodavyn). ²⁹Si and ²⁷Al MAS NMR results suggested that Al³⁺ cations tended to occupy the tetrahedral sites on tetrahedral sheets. The new findings of this research have great significance for understanding the stability of hydrotalcite, heterogeneous nucleation and growth of hydrotalcite and silicate minerals as well as the formation processes of the hydrothermal saponite deposits.