The invention discloses a surface-modified aluminum hydroxide and a preparation method thereof. The surface-modified aluminum hydroxide comprises, by mass, 30 parts of aluminum hydroxide and 0.3-1.2 parts of a modifier. The modifier is Span, a complex of Span and stearic acid, or a complex of Span and sodium stearate. The Span is Span 40, Span 60 or Span 85. The preparation method comprises the following steps of preparing an aluminum hydroxide aqueous suspension having a mass concentration of 10%, heating the aluminum hydroxide aqueous suspension to a temperature of 80-85 DEG C, preparing a modifier-ethanol solution having a modifier mass concentration of 1%, adding the modifier-ethanol solution into the aluminum hydroxide aqueous suspension, heating the mixed suspension to a temperature of 70-90 DEG C, keeping the temperature for 0.5-1.5h, and carrying out standing, suction filtration, washing, drying and grinding so that the surface-modified aluminum hydroxide which is conducive to uniform dispersion in a polymer base and improves base compatibility is obtained.