The invention discloses a method for preparing cationic polyacrylamide emulsion in an aqueous dispersion polymerization manner. The preparation method comprises the following steps: dissolving nonionic monomers, cationic monomers, a dispersion stabilizer, parital inorganic salts and additives in deionized water, stirring in a constant-temperature water bath at 40 to 70 DEG C to form a homogenous system, introducing nitrogen to remove oxygen, adding an initiator accounting for 50 to 80 percent of the total weight of an initiator raw material, continuously reacting for 3 to 6 h, adding the remaining initiator, then reacting, and adding the remaining inorganic salts, continuously stirring, thus obtaining the cationic polyacrylamide emulsion. A novel oxidization reduction initiator system is applied, the graphene oxide is used as a reducing agent, persulfate is used as an oxidant, and the cationic polyacrylamide emulsion is prepared by virtue of the aqueous dispersion polymerization method. The emulsion prepared by the method has the characteristics of instant solubility, high molecular weight, good mobility, good stability, no organic solvent in the polymerization process, environmental friendliness, nontoxicity, capability of adsorbing trace heavy metal ions and the like.