The invention relates to a production method of monocrystalline aluminum hydroxide, in particular to a method for producing monocrystalline aluminum hydroxide by decomposing aluminate steel fine liquid in the production process of aluminum oxide. The method is characterized by comprising the following steps: (1) adding sodium oxalate into the sodium aluminate fine liquid to carry out decomposition reaction, filtering the decomposition product, and sufficiently washing the product with hot water to obtain seed crystals; and (2) adding the seed crystals obtained in the step (1) into the sodium aluminate fine liquid to carry out decomposition reaction, filtering the reaction product, and washing the product to obtain the monocrystalline aluminum hydroxide product. The monocrystalline aluminum hydroxide product prepared by the production method provided by the invention is transparent, has the characteristics of high hardness, uniform granularity, low oil absorption rate and the like, is micropowder filler with good properties, and can be used as the precursor of monocrystalline aluminum oxide granules; and the high chemical purity, granularity and physical and chemical properties of the monocrystalline aluminum hydroxide product can satisfy the production requirements of non-metallurgical level aluminum oxide, thus effectively lowering the production cost of non-metallurgical level aluminum oxide.