The invention provides a method for preparing sulfate-resistant polycarboxylate water-reducing agents and application thereof. The sulfate-resistant polycarboxylate water-reducing agents comprise monomers a, monomers b and monomers c which are subjected to reversible addition-fragmentation chain transfer polymerization. The method and the application have the advantages that strong adsorption groups and phosphate groups are introduced into block polycarboxylate main chain structures definitely distributed on sequence structures, accordingly, block polycarboxylate is high in adsorption capacity, and the sulfate resistance of the block polycarboxylate can be improved; the sulfate-resistant polycarboxylate water-reducing agents are low in dosage and slump loss, high in water-reducing rate and good in cement adaptability and is sulfate-resistant, the method for synthesizing the sulfate-resistant polycarboxylate water-reducing agents is simple, is low in production cost and little in environmental pollution and has low technological requirements, and the like.