The invention discloses a polycarboxylic acids water reducing agent containing low molecular weight carbon dioxide copolymer, which is a copolymerization product of unsaturated large monomer, polyethylene glycol monomethyl ether monounsaturated acid ester, cyclic monomer and other unsaturated monomer containing low molecular weight carbon dioxide copolymer, wherein the low molecular weight carbondioxide copolymer is the copolymer of the carbon dioxide and the epoxy compound. In the invention, the low molecular product generated by the copolymerization reaction of the carbon dioxide and the epoxy compound is used. According to the reasonable design of the molecular structure, the low molecular carbon dioxide copolymer is esterified for blocking to generate the unsaturated large monomer, and then the novel polycarboxylic acids water reducing agent with long/short functional branch is prepared through the copolymerization of the unsaturated large monomer and other functional monomer. The polycarboxylic acids water reducing agent for concrete has stable product quality, and the preparation method has the advantages of short process, simple technology, easily controlled reaction condition, and easy industrial production.