The invention relates to low-viscosity filler aluminum hydroxide and a preparation process and system thereof, and belongs to the technical field of non-metallurgical alumina preparation. The processfor producing the low-viscosity filler aluminum hydroxide comprises the following steps: raising air temperature in a vertical mill until the air temperature in the vertical mill is 150-170 DEG C, andcrushing an aluminum hydroxide raw material in the vertical mill during the temperature raising process to obtain the low-viscosity filler aluminum hydroxide. The method saves energy, is environmentally friendly and is easy to operate. The air temperature is raised by using a mode of steam heat exchange, the temperature of steam is not more than 180 DEG C, and the temperature of air subjected totemperature raising is not more than 170 DEG C when the air is in contact with the aluminum hydroxide raw material, so that the aluminum hydride has no risk of losing crystal water, and quality is improved.