The invention discloses a method for extracting metal hydroxide from gangue. The method comprises the following steps of: crushing the gangue which is taken as a raw material, adding solution of sulfuric acid and water, heating and reacting to obtain solid-liquid mixed solution, filtering and separating, deslagging, extracting alum from recovered filtrate, separating to obtain the filtrate, adding calcium oxide or calcium hydroxide into the filtrate, controlling the pH value of the solution to be between 3.2 and 6.5, precipitating and filtering, and removing filtrate to obtain the metal hydroxide. No smoke and waste gas are emitted, the method is environment-friendly and pollution-free, waste resources are utilized, and production cost is low.