In order to improve the selective adsorption, the cotton fiber was modified. First, polydopamine (PDA)-Fe3O4 magnetic cotton fiber was formed by polymerizing dopamine hydrochloride (DA) and nano-ferric oxide (Fe3O4) in Tris-HCl buffer solution. Secondly, dodecyltriethoxysilane (DTES) was used to further react PDA-Fe3O4 cotton fiber in alkaline ethanol aqueous solution to obtain DTES-PDA-Fe3O4 magnetic-superhydrophobic cotton fiber. FTIR, XRD, TG, SEM, EDS, AFM, water contact angle measuring instruments were used to characterize the chemical composition, surface microstructure, and hydrophobic properties of cotton fibers before and after modification. The separation efficiency, and adsorption properties also be tested. The results show that DTES-PDA-Fe3O4 cotton fiber had a rough structure of micro/nano size. The synthesized cotton fiber had magnetic with excellent superhydrophobicity and a water contact angle greater than 160°. It also had ultra-high selective adsorption, oil-water separation efficiency and reusability. It can absorb chloroform 8.96 times its own weight, and the separation efficiency of chloroform oil-water mixture could reach above 98.90%.