The invention is applicable to the field of compound preparation and provides a preparation method of aluminum phosphate. The preparation method comprises the following steps: grinding coal gangue into fine particles, and enabling the fine particles to react with a hydrochloric acid solution in a mass ratio of 1 to 2.15 in a corrosion-resistant container; filtering a generated mixed solution of aluminum chloride, ferric chloride and a small amount of chloride, so as to obtain a filter cake and filtrate, and purifying the filtrate by virtue of aluminum hydroxide, so as to generate aluminum chloride and hydroxide precipitates; filtering the purified filtrate, so as to obtain a filter cake containing hydroxide and an aluminum chloride settled solution; enabling the aluminum chloride settled solution to react with phosphoric acid in a mass ratio of 1 to 0.73 in the corrosion-resistant container; and filtering aluminum phosphate precipitates and a hydrochloric acid solution generated from the aluminum chloride solution and phosphoric acid, and washing, spin-drying, drying and grinding filtered aluminum phosphate, so as to obtain aluminum phosphate powder. According to the preparation method, the pollution of coal gangue is effectively eliminated, the social benefit is gained, furthermore, the production costs of aluminum phosphate and iron phosphate are lowered, and the economic benefit is increased.