Abstract: Nano magnesium hydroxide was coated on the surface of fly ash by non-uniform nucleation method to carry out inorganic modification. The influence of different magnesium hydroxide coating amount (\mathrmm_\mathrmMg(\mathrmOH)_2 /m_{fly ash}) on the coating effect of composite powder was investigated. The structure, surface functional groups, morphology and stability of the composite powder were characterized by SEM, XRD, FTIR and TG. Nylon 6 composite was prepared by filling nylon 6 with composite powder. The mechanical properties, thermal deformation temperature, melt index and flame retardancy of the composite were tested. The results show when the coating content is 70%, the surface of fly ash is coated with nano magnesium hydroxide with regular morphology and uniform size, and the specific surface area of the composite powder increases from 1.72 m²/g to 56.62 m²/g. The mechanical properties and flame retardancy of nylon 6 composites are better than those of pure nylon 6. Fly ash-based composite powder can replace pure magnesium hydroxide to fill nylon 6, and solve the problem of poor compatibility and dispersion when fly ash and pure magnesium hydroxide are used as fillers alone.