The invention relates to a composite coagulant for advanced treatment of biochemical tail water, as well as a preparation method and application thereof. The inorganic polymer composite coagulant containing boron, silicon, aluminum and iron, which is prepared by adopting an iron source, a boron source and a silicon-aluminum source as raw materials, is applied to coagulation purification processing coagulant of all kinds of waste water, and is particularly applied to advanced treatment of the biochemical tail water. The iron source, the boron source and the silicon-aluminum source are adopted as the raw materials; the inorganic polymer composite coagulant is prepared through alkaline hydrolysis, acid leaching oxidation, hydrolysis and polymerization under heating; and the prepared composite coagulant contains 20 percent-35 percent of silicon, 15 percent-25 percent of aluminum, 6 percent-11 percent of iron, and 3 percent-6 percent of boron. The coagulant has easy-to-get raw materials and concise process, and is economic and applicable. The obtained inorganic polymer composite coagulant has good capacities of absorption and bridging, charge neutrality, bonding aggregation and decoloration, and less dosage, and is widely applied to waste water treatment field, in particular to advanced treatment of the biochemical tail water; and the flocs are big and are fast in sedimentation velocity.