The invention discloses a slow-release polycarboxylate 
superplasticizer and a preparation method thereof. The preparation method comprises specific steps as follows: firstly, small unsaturated 
carboxylic acid monomers and diphenylphosphinyl 
chloride are subjected to a partial reaction, and small unsaturated monomers containing 
phosphate ester groups are obtained; then 2-phenylethylamine and 
phosphate ester groups are added to a 
reaction system and subjected to a reaction, and small unsaturated monomers containing 
amide groups are obtained; the small unsaturated monomers containing the 
amide groups are subjected to an oxidation-reduction free 
radical polymerization reaction with unsaturated polyether macromonomers under the action of an initiating agent, a 
reducing agent and a 
chain transfer agent, and a copolymerization product is obtained; pH value is adjusted to 6-7 after the reaction, water is added, and the polycarboxylate 
superplasticizer is obtained. The 
amide groups containing 
benzene rings are introduced to side chains of the prepared polycarboxylate 
superplasticizer, so that the dispersibility of the polycarboxylate superplasticizer for 
cement is improved, the polycarboxylate superplasticizer has good 
slump retaining performance on the premise of water reduction, and besides, the yield of the polycarboxylate superplasticizer containing the amide groups can reach 94% or higher.