The work aims to improve the dispersivity and compatibility of nano-sized aluminum trihydroxide (Nano-ATH) flame retardant modified in chemical grafting method instead of traditional physical adsorption method. Wet surface modification of Nano-ATH was carried out with titanate coupling agent UP-311. Effects of coupling agent usage, modification temperature and modification time on particle size of Nano-ATH were studied in orthogonal experiment method. The modification effect on Nano-ATH prepared under optimal modification conditions was evaluated by such analytical means as infrared spectroscopy, particle size and Zeta potential, respectively. The optimal modification conditions were: m (modifier):m (Nano-ATH) as 0.03:1, modification temperature as 75 ℃, and modification time as 30 min. Nano-ATH could be bonded with titanate coupling agent UP-311 by virtue of chemical bond. Surface property of the Nano-ATH powder is changed obviously after modification, the hydrophile-oleophobic Nano-ATH turns into lipophilic-hydrophobic Nano-ATH, and the purpose of surface modification is achieved.