The invention provides a viscosity-reducing type polycarboxylic acid water reducing agent for high-strength concrete, and belongs to the technical field of water reducing agents. The viscosity-reducing type polycarboxylic acid water reducing agent has the following general formula as shown in the description, wherein R' is selected from CH3 or H; R1 is selected from CH2CH2 or CH3CHCH2; R2 is small-molecule alcohol or alcohol amine with a hydroxyl group; a, b, c, m, n and g are all integers; a, b and c are taken from 1 to 50; m is taken from 5 to 150; n is taken from 20 to 110; and g is taken from 1 to 12. According to the viscosity-reducing type polycarboxylic acid water reducing agent for high-strength concrete, maleic anhydride, polyethylene glycol or polypropylene glycol, the small-molecule alcohol or alcohol amine and a phosphorylating reagent are adopted to prepare maleic anhydride modified phosphate ester for polycarboxylic acid water reducing agent synthesis, so that a phosphategroup of a specific structure is introduced into a molecular structure of the polycarboxylic acid to increase the polymer adsorption capacity and lower an LHB value; an effect of reducing the viscosity of the concrete is achieved by improving the capacity of a surfactant of the water reducing agent and weakening the interactivity between adsorption interfaces. Meanwhile, a functional monomer preparation process can be simplified and the production cycle can be shortened; and no expensive monomers are needed, and the production cost is reduced.