The invention relates to an anionic acrylic emulsion containing a cationic monomer and a preparation method of the emulsion. Based on the dry weight of the emulsion, the anionic acrylic emulsion is prepared from a monomer comprising the following components: 80-99 wt% of an acrylate monomer, 0.1-10 wt% of functional monomer acid, 0.1-10 wt% of a cationic monomer and 0.1-5 wt% of an emulsifier. Thepreparation method adopts a step-by-step emulsion polymerization method and includes, firstly, polymerizing a pre-emulsion containing the functional monomer acid; then neutralizing with a neutralizing agent; and then polymerizing the pre-emulsion containing the cationic monomer. The preparation method can be carried out on normal production equipment of the anionic acrylic emulsion without additional equipment. Due to the introduction of the cationic monomer containing amino groups in the anionic acrylic emulsion, a water-based wood sealing primer prepared by the anionic acrylic emulsion hasexcellent tannic acid sealing performance, and overcomes the limitation of raw material selection and construction inconvenience in preparing the primer by the cationic acrylic emulsion.