Abstract: Provided are a mercapto group-loaded layered double hydroxide (LDH)-based magnetic composite particle, and a preparation method and use thereof. The method includes: dissolving a water-soluble divalent magnesium salt and a water-soluble trivalent aluminum salt in water to obtain a mixed salt solution; pumping ammonia water into the mixed salt solution, and performing reaction I under stirring, subjecting an obtained product to centrifugation, and washing product obtained with deionized water; mixing the product with water, ethanol, and a coupling agent KH-580 to obtain a mixture, subjecting the mixture to reaction II while stirring to obtain mercapto group-loaded LDH; mixing the mercapto group-loaded LDH with ferroferric oxide and sodium alginate, adding deionized water thereto, and stirring to obtain a homogeneous mixed solution; adding the homogeneous mixed solution into a calcium chloride solution dropwise to form a magnetic bead; and leaving the magnetic bead to harden, and then washing and drying.