The invention provides a preparation method of a viscosity-reducing polycarboxylate superplasticizer and an application thereof. The viscosity-reducing polycarboxylate superplasticizer can remarkably reduce the viscosity of concrete on the premise of guaranteeing high water reduction and high slump loss resistance; and the preparation method has the advantages of simple technology, low cost, little pollution and the like. In the preparation method, a specific unsaturated acid monomer, a specific unsaturated polyether macromonomer and N-(4-vinyl benzyl)-N, N-dialkylamine in a molar ratio of (2.5-6.5):1:(0.05-0.2) are subjected to free radical copolymerization reaction to obtain the viscosity-reducing polycarboxylate superplasticizer.