perfluorobutyl acrylate copolymer(PC4FA) emulsion having short carbon chain is prepared by the semi-continuous seed emulsion polymerization with acrylate monomer containing a short perfluoroalkyl chain 2-(N-methyl perfluorobutyl sulfonamido) ethyl acrylate(C4), stearyl methylacrylate(SMA), methyl methacrylate(MMA) and vinylidene chloride(VDC) as the monomer, and then the copolymer emulsion is applied to fabric finishing. The structure and surface element composition of the copolymer were investigated by FT-IR and XPS. The AFM and SEM were used to observe the cotton fabric surface treated with and without perfluorobutyl short carbon chain acrylate water-repellent agent. The influences of dosage of fluorinated acrylate monomer, the concentration of water-repellent finishing agent, curing temperature and time on water repellent property of cotton fabric were investigated. The optimum finishing process was: the mass content of fluorinated acrylate monomer was 51.02% of the copolymer, the mass fraction of finishing agent was 1.8%, curing temperature was 180℃, curing time was 5min. The finished cotton fabric with above process had excellent water repellency property with water contact angle up to 142.5°, water repellent grade was 100.