The invention is applicable to the field of compound preparation and provides a preparation method of aluminum chloride. The method comprises steps as follows: coal gangue is crushed into fine particles which react with a hydrochloric acid solution in the mass ratio being 1:2.15 in a corrosion-resistant container; a mixed solution which contains aluminum chloride, ferric chloride and a small quantity of chlorides and is produced by the coal gangue and the hydrochloric acid solution is filtered, and a filter cake and a filtrate are obtained; the filtrate is purified with aluminum hydroxide, and aluminum chloride and hydroxide precipitates are produced; the filtrate purified with aluminum hydroxide is filtered, and a white filter cake and a clear aluminum chloride solution are obtained; the clear aluminum chloride solution is distilled, cooled and crystalized, and aluminum chloride crystals are obtained. Aluminum chloride and ferrous chloride are prepared through coal gangue acidification, the coal gangue of coal mines is effectively treated, the social benefit is created while pollution is eliminated, besides, the production cost of aluminum chloride and ferrous chloride is reduced, and the economic benefit is improved.