The invention discloses a preparation method for a concrete water reducer with ultrahigh water reducing performance. The preparation method comprises the steps of performing thermal induced polymerization on unsaturated carboxylic acid monomers A, unsaturated phosphoric acid monomers B, ultraviolet active special monomers C and initiation assistant monomers D in a toluene solution by adopting an initiator E to obtain macromolecules F with ultraviolet activities; and adding the prepared solid active macromolecules F into de-ionized water for dissolution, and under the ultraviolet irradiation, slowly dropwise adding N-vinyl pyrrolidone (N-VP) and the monomers A to perform a graft polymerization reaction to obtain final water reducer molecules. According to the water reducer prepared with the method, a side chain has double stabilization effects of electrostatic repulsion and steric hindrance, so that compared with a conventional comb polycarboxylic acid water reducer, the water reducing capability of the concrete water reducer is greatly improved.