The invention discloses a preparation method of an ester-type viscosity-reducing polycarboxylic acid superplasticizer. The method comprises the following steps: (1) preparing an unsaturated highly-branched small monomer; (2) carrying out an esterification reaction; and (3) carrying out a copolymerization reaction. According to the method, the unsaturated highly-branched small monomer is synthesized, and introduced into the polycarboxylic acid superplasticizer together with a benzene-containing rigid small monomer methyl 4-vinylbenzoate, so that the molecular conformation of the polycarboxylicacid superplasticizer can be more extended, the steric hindrance effect of side chains of the polycarboxylic acid superplasticizer is further improved, and thereby the viscosity of concrete is greatlyreduced. The preparation process is simple, the production is convenient, the cost is low, and the pollution is small.