Loose and non-reunite ultra-fine powder of aluminum hydroxide with size less than 100nm are prepared from rich-alumina residues by using a new dispersant.Here modification of mineral composition is adopted to activate Al2O3 in rich-alumina residues.Disturbing factors of crystal change of C2S are eliminated effectively and the complete self-pulverization of rich-alumina residues sinter is realized.Aluminum in the form of NaAlO2 is extracted from the activated stuff by using 8% sodium carbonate solution.It is studied in experiments that the effects of the dispersant's molecular weight,adding method,amount and the concentration of sodium aluminate solution on the preparation of ultra-fine aluminum hydroxide and the best technique parameters are achieved.The dispersion mechanism of the new dispersant during the preparation process is also discussed.