The invention discloses a low-viscosity preparation method of cationic polyacrylamide copolymer water-in-water dispersion liquid. The key point of the method is as follows: a dispersion liquid of a seed polymer dissolved with monomers is prepared before synthesis of the dispersion liquid, and then the dispersion liquid is prepared through polymerization, wherein the seed polymer can be subjected to phase splitting in a salt solution to form micro particles namely 'seeds'. As the polymerization process takes seed internal polymerization as the primary from the beginning, the viscosity of the system in the whole polymerization process can be effectively reduced, and the storage stability of the formed water-in-water dispersion liquid can not be easily influenced by the temperature. The preparation method of the dispersion liquid is suitable for the industrial production of the water-in-water dispersion liquid, and is especially suitable for the preparation of the cationic polyacrylamide copolymer water-in-water dispersion liquid with low and high degree of ions.