The invention relates to a preparation method for instant cation polyacrylamide with a high molecular weight. The monomer of acrylamide and a cation type monomer are used as the main materials, firstly, the acrylamide and a comonomer are dissolved into deionized water; then a photoinitiator and / or a photosensitizer additive and a redox initiator are added in the solution to form a compound initiating system; the materials of the components are uniformly mixed in a viscous state, poured on a plane and paved into a sheet shape, introduced with high purity nitrogen, then ultraviolet light illumination is carried out to the materials paved into the sheet shape; after the materials of the viscous state are in a gel block, the materials are dried, broken, granulated and cracked to obtain a product of a dry powder shape. In the invention, the heat release and temperature rising of bulk adiabatic polymerization are avoided, while the polymerization and drying are carried out under a lower temperature; therefore, the reaction process is stable; the product has the advantages of higher molecular weight, better solubility, faster dissolving speed, simple technique and low cost; in addition, the processing after polymerization reduces the working procedure of breaking of bulk gel.