The invention relates to a novel preparation method of reverse-phase emulsion polymer, which comprises the preparation of water-in-oil reverse-phase emulsion and the polymerization of water-soluble vinyl monomer in at least one water-in-oil emulsion. The method comprises the following steps in detail: (1) the preparation of monomer water solution; (2) dewatering continuous phase and emulsifying surfactant; (3) preparing reverse-phase pre-emulsion; and (4) polymerizing reverse-phase emulsion. Compared with the prior art, the invention creatively adopts a ray-radiation grading continuous-polymerization process, is easy to realize the control to the conversion ratio by the grading polymerization and avoids the instant release of the heat of polymerization; and moreover, the invention can realize the low-temperature continuous polymerization, not only has high molecular weight and low residual monomers of products, but also has safe and environmental-friendly process operation and high production efficiency.