The invention provides cationic polyacrylamide aqueous emulsion and a preparation method of cationic polyacrylamide aqueous emulsion. The method for preparing the aqueous emulsion comprises the steps of preparing a reaction type dispersing agent; and balancing the free radical concentration through a multistage complex initiation system under the participation of a salt liquid system and the dispersing agent, and carrying out dispersion polymerization of acrylamide and cationic monomer aqueous emulsion. The aqueous emulsion can be dissolved rapidly, has high molecular weight, good mobility and high stability, and can be widely applied to the water treatment fields of all industries.