A gamma-AlOOH nanocrystal powder able to be automatically dispersed in aqueous and some organic solvents for preparing self-dispersing superfine non-toxic aluminum hydroxide flame-retarding agent, or the high-molecular polymer/inorganic nano-material composition, or the microcrystalline alumina ceramics or high-performance alumina ceramics as grinding medium is prepared through reaction of sodiumaluminate or Al salt solution in alkaline medium, extracting the generated deposit, adding peptizing agent and disperser, peptizing and dispersing to obtain aluminum hydroxide colloid, reacting on additive and surfactant for surface modifying, and spray drying.