The invention relates to a bio-adsorbent prepared by modifying ball-milled tea residues and a preparation method and application of the bio-adsorbent. The method includes the steps of removing dissoluble pigments and other substances in tea residues through sulfuric acid to obtain pigment-removed tea residues, ball-milling pigment-removed tea residues to obtain superfine tea residues, soaking superfine tea residues in a mixed solution of metal ions of zirconium, iron, lanthanum, cerium, titanium and the like, loading generated hydroxide in tea residues under the alkaline conditions, conducting high-temperature drying to ensure that hydroxide can be sufficiently loaded in superfine tea residues, and finally conducting washing and drying to prepare the defluorination bio-adsorbent. Tea residues serve as the main raw material of the bio-adsorbent, the resource is wide and the price is low; not only can tea residue waste and pollution caused by tea residue waste be eliminated, but also a new approach is provided for high-value utilization of tea residues, and the problem that it is difficult to dispose tea residues in the industrial production process is solved. The bio-adsorbent is mild in reaction condition in the preparation process, the process route is simple and easy to implement, and industrial operation is easy.