Abstract: A one-step method for preparing a magnetic magnesium-iron layered double hydroxide (LDH)-biochar composite material and use thereof provided. Biomass, as a substrate, is placed in a ferric salt solution, magnesium hydroxide is added, the materials are fully stirred and aged for a certain time, the aged materials are dried to obtain a magnesium-iron LDH-biomass, and the magnetic magnesium-iron LDH-biochar composite material is obtained after pyrolysis. The method solves problems of uncontrollable reaction and low crystallinity of products in preparing LDH using a coprecipitation method, reduces the amount of drugs, omits a step of adjusting a pH with an alkaline solution, improves yield and reduces a cost. The obtained magnetic magnesium-iron LDH-biochar composite material exhibits an excellent performance in adsorbing phosphate in water and can be recovered by an external magnetic field. Thus, an important method is provided for preparing LDH and the composite thereof.