The invention discloses an apparatus used for preparing high-activity calcium hydroxide by using carbide slag alkali residue. The apparatus comprises a sodium bicarbonate metrical adding apparatus, a carbide slag adding apparatus, and sequentially connected components of a mixing tank, a carbide slag delivering screw conveyor, a drier, a crusher, an air separator, a deduster, and a collection chamber. The sodium bicarbonate metrical adding apparatus and the carbide slag adding apparatus are both connected with the mixing tank. The invention also relates to a method used for preparing high-activity calcium hydroxide by using the carbide slag alkali residue. According to the invention, sodium bicarbonate and the carbide slag are stirred in the mixing tank; the mixture is processed by using the drier, the crusher, and the air separator; and a calcium hydroxide finished product can be collected by using the deduster. The apparatus and the method provided by the invention are advantaged inthat: an advanced micro-porous structure is formed in the calcium hydroxide particles, such that the specific surface area of calcium hydroxide is increased, and the reaction activity is improved. When calcium hydroxide is applied as a desulphurization absorbent, the desulphurization efficiency can be improved by 5-10%, such that the consumption amount of absorbent can be reduced, and the cost ofdesulphurization operation can be reduced.