The invention provides a comb-type polymer. The comb-type polymer is a copolymer with weight-average molecular weight of 20000-50000 prepared from monomers A, B and C, wherein the monomers A comprise two or more nonsaturated alkenyl-terminated polyether large monomers, the monomer B is 2-acrylamide-2-methylpropanesulfonic acid and the monomer C is unsaturated carboxylic acid or its derivative small monomer. The invention also provides a preparation method of the comb-type polymer and a use of the comb-type polymer as a polycarboxylic acid water reducer. Through combination of three types of the monomers, comb-type polymer polymerization is thorough, a monomer conversion rate is high and water reducer performances of the polycarboxylic acid water reducer are substantially improved.