Abstract: The sizing agent for acetate filament was synthesized by using vinyl acetate, methacrylic acid and butyl acrylate as monomers and adopting semi-continuous emulsion polymerization method. Solid content of the polymer, viscosity of size pastes conversion rate of monomers and water resistance of sizing film were tested. Then using single factor analysis to optimize these influencing factors, the best synthesizing process was established. Besides, the structure and properties of the polymer were analyzed by FTIR. Experimental results indicate that the property of copolymer emulsion is excellent with 0.75% initiator (APS, based on the total monomer), 3% complex emulsifier (OP-10:SDS is 3:1) and 4 hours of dropwisetime.