Mullite nanocomposites powders were successfully synthesized from high-aluminium coal fly ash, coal gangue, andalusite-sericite phyllite and oil shale slag through the hydrothermal crystallization process after the removal of the residual carbon, deferrization or calcination. The phase constitution and microstructure of the synthesized mullite nanocomposites powders were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and energy-dispersive X-ray (EDX). Moreover, the reaction mechanism was analyzed according to MAS-NMR results. The experimental results show that mullite nanocomposites powders can be prepared from any silicon-aluminium-bearing minerals at 80～90℃ for 3 h with NaOH concentration of 2～4 mol/L and L/S ratio (liquid/solid ratio in mass) of 10 mL/g. Mullite nanocomposites which are synthesized from coal gangue, andalusite-sericite phyllite and oil shale slag are mainly existent as whole puncheon-shaped crystals, whose average lengths are 50 nm, 300 nm and 150 nm, and whose average aspect ratios are 4, 3.5 and 3.75, respectively. Otherwise, when coal fly ash is used as the raw material, mullite nanocomposites are mainly rod-like, acicular and fibroid in shape with the length in the range of 30～80 nm and the diameter in the range of 4.5～30 nm. MAS-NMR analysis indicates that the nano-size mullite grows around the central atoms Al, Si of the [AlSi]O₄ tetrahedron and the growth unit Al[OH]₄^- is existent in this process definitely.