The chemical composition of surface sizing agent BLD-02 was researched, as well as its synthesis method. BLD-02 was analyzed by infrared spectroscopy, and the main monomers were styrene, acrylate and acrylonitrile. In the process of synthesis, these monomers were prepared for copolymer latex by seed emulsion polymerization. Through a series of pre-trial, the results showed that when the contents of initiator and emulsifying agent were 0.3% and 3% (the percentage of total) respectively, the weight ratio of hard monomer to soft monomer was 2/3 and the reaction temperature was 75°C, the synthetic emulsion with higher viscosity, smaller particle size and well-dispersion was prepared. Results showed that the infrared spectrum and properties of the synthetic emulsion, to some extent, were as same as that of BLD-02.