Nano-sized highly-dispersed hexagonal magnesium hydroxide as the flame retardant was prepared from hydromag-nesite by a simple synthesis route by calcination-hydration-calcination-hydrothermal method.After the initial process was con-firmed,the effects of magnesium oxide（MgO） dosage,hydrothermal temperature and hydrothermal time on the crystallinity and morphology of Mg（OH）₂ were investigated.Finally,the optimized hydrothermal conditions were determined as follows：MgO dosage was 10%~25%（mass fraction）,hydrothermal temperature was 150 ℃ and hydrothermal time was 3 h.Polyvin-ylpyrrolidone（PVP）,cetyltrimethylammonium bromide（CTAB）,polyethylene glycol 6000（PEG6000） and polyethylene gly-col 200（PEG200） were separately added to the reaction system during the hydrothermal process to explore the effects of dif-ferent modifiers and mass fractions of the modifier on the crystallinity and dispersibility of Mg（OH）₂ nanoparticles.Result showed that the dispersibility of Mg（OH）₂ nanoparticles can be well improved using 4%（mass fraction） PVP and finally hexa-gonal nano-Mg（OH）₂ with high dispersibility,complete crystal shape,uniform particle size,300~400 nm diameter and 40~60 nm thickness was obtained.