The invention discloses a viscosity-reducing fluorescent polycarboxylate water reducer and a preparation method thereof. The method adopts an unsaturated acid monomer and organic molecules with hydroxyl and fluorescent properties to carry out esterification reaction to obtain unsaturated monomers with fluorescent properties, and then performing oxidation-reduction free radical polymerization reaction on the unsaturated monomers with fluorescent properties, unsaturated polyether monomers, unsaturated acid monomers and derivatives thereof to obtain the viscosity-reducing fluorescent polycarboxylate water reducer. The viscosity-reducing fluorescent water reducer prepared by the method retains the advantages of the existing water reducer, has good controllability of the production process, fewside reactions, strong adaptability to different cement varieties, high water reducing rate, and higher cost performance and competitive advantages. Concrete mixed with the viscosity-reducing fluorescent polycarboxylic acid water not only has the pressure resistance and heat preservation of traditional building materials, but also has viscosity-reducing and fluorescent properties, and the characteristics of the concrete are further expanded.