A reducing agent monomer for preparing a styrene-acrylic emulsion by an oxidation-reduction reaction at room temperature and a synthesis method thereof are disclosed. Maleic anhydride (MAH) and dimethylethanolamine (DMEA) are used as raw materials to synthesize the reducing agent monomer: 4-(2-(dimethylamino)ethoxy)-4-oxobut-2-enoic acid, and the synthesis method involves inexpensive easily-available raw materials, simple synthesis conditions, and easy purification. With the synthesized reducing agent monomer as a reducing agent, potassium persulfate (KPS) as an oxidizing agent, water as a dispersion medium, sodium dodecyl sulfate (SDS) as an emulsifier, and styrene, butyl acrylate (BA), and methylmethacrylate (MMA) as comonomers, free-radical microemulsion polymerization is conducted at room temperature to obtain a styrene-acrylic emulsion. In the synthesis of the styrene-acrylic emulsion, a monomer conversion rate is high, and a styrene-acrylic emulsion with a high molecular weight and a branched structure can be obtained at room temperature.