The invention discloses a preparation method of an amphiphilic fluorine-containing gradient copolymer, and the method is used in oil-water separation ultrafiltration membrane antifouling modification. First, with a reversible addition fragmentation chain transfer (RAFT) controlled free radical polymerization method, with a semi-continuous feeding manner, an amphiphilic fluorine-containing gradient copolymer is prepared; the prepared amphiphilic fluorine-containing gradient copolymer is adopted as a modifying agent, and is used for modifying the ultrafiltration membrane with a blending modification method, wherein the membrane is prepared with a classic non-solvent induced phase transformation method. The surface of the membrane modified with the amphiphilic fluorine-containing gradient copolymer has an amphiphilic hierarchical structure with the coexistence of hydrophilic and hydrophobic regions. The amphiphilic fluorine-containing gradient copolymer preparation method is simple; and the modified membrane preparation process has the advantages of easy operation and mild conditions. The modified membrane has good oil-water separation performance and excellent antifouling effect, and can be used in water treatment and especially the separation purification of oil-containing wastewater.