The invention relates to an aluminium hydroxide drying system and a drying method thereof. The method comprises the steps that wet aluminium hydroxide is dried through a Venturi drier and a second-stage preheating cyclone separator from a gaseous suspension roaster; then, after being cooled through a fluidizing cooler, a part of dry aluminium hydroxide enters a storage bin; a bag type dust collector and an electrical dust collector are utilized at the same time to recycle solids in the gas; and the other part of dry aluminium hydroxide enters a roasting main furnace through a second-stage preheating cyclone separator to be roasted, and the roasted dry aluminium hydroxide enters a third-stage preheating cyclone separator to be subjected to solid-gas separation. According to the aluminium hydroxide drying system and the drying method thereof, hot flue gas generated from roasting is utilized, so that compared with a traditional drying technology, the drying method is low in investment and is more energy-saving.