Novel water-soluble macromolecular UV absorber PVAm-g-BP-4 was synthesized with 2-hydroxybenzophenone as UV absorbing group and polyvinylamine (PVAm) as backbone. PVAm-g-BP-4 was synthesized through 2 steps. Reactive intermediate 2-hydroxy-4-methoxy-5-chlorosulfonylbenzophenone (CBP-4) was first synthesized from 2-hydroxy-4-methoxy-5-sulfo-benzophenone (BP-4) with thionyl chloride as chlorination agent and solvent and N, N-dimethylformamide (DMF) as catalyst, the yield of the intermediate could reach 94.5%. And then PVAm-g-BP-4 was synthesized by grafting CBP-4 onto PVAm. CBP-4 was confirmed to be grafted onto PVAm through TLC, IR and UV spectroscopy. The preparation conditions of the intermediate and product were optimized by studying the dosage of catalyst, reaction temperature, reaction time, solvents ratio, etc. Application of PVAm-BP-4 on cotton fibers shows it provides the fibers good anti-UV property and its wash fastness was satisfactory.