The invention discloses a method for synthesizing an environment-friendly surface sizing agent by a soap-free emulsion method, and the method comprises the following steps: mixing a polymeric dispersant, (methyl) styrene, a (methyl) acrylate monomer and a functional cationic monomer with water, heating to 80-90 DEG C, then adding an initiator to react, and fully reacting to obtain the environment-friendly surface sizing agent, wherein the polymeric dispersant is formed by polycondensing maleic anhydride and polyvinyl alcohol and then reacting with chlorosulfonic acid. The environment-friendlysurface sizing agent is a high-performance and environment-friendly product capable of meeting the requirements of paper surface sizing. The product can significantly improve the water resistance of the paper by surface sizing of base paper together with starch. At the same time, the method can also improve the strength of paper.