The invention discloses aluminum hydroxide and a preparation method thereof. The aluminum hydroxide is prepared by a carbonization method and contains over 94 weight percent of pseudo-boehmite. The method comprises the following steps: adding a small amount of organic pore-expanding agent and defoaming agent into aqueous solution of sodium metaaluminate; introducing a mixed gas of air and CO2 into the solution to stir the solution to form one-step gel; generating evenly grained pseudo-boehmite; and roasting the evenly grained pseudo-boehmite to obtain alumina with a large pore volume, a largeaperture and concentrated pore distribution. The aluminum hydroxide is suitable to be used as a component of absorbent or catalyst carrier, particularly a component of a heavy oil or residual oil hydrodemetallization catalyst carrier.