The invention discloses a low air-entraining viscosity reducing polycarboxylic water reducer which is formed by free radical polymerization of an alkyl phosphotriester mixture, TPEG and unsaturated carboxylic acid under the action of an initiator and a chain transfer agent. By introducing an alkyl phosphotriester monomer containing two double bonds to a main chain, molecules of the obtained polycarboxylic water reducer are of heterocyclic multibranched structures, so that the thickness of a polycarboxylic water reducer adsorption layer is increased, the plastic viscosity of concrete can be remarkably reduced, and meanwhile, by introducing the phosphotriester structure into the molecules of the polycarboxylic water reducer, the foaming property of the obtained polycarboxylic water reducer can be effectively improved and the air entraining property thereof can be reduced. The low air-entraining viscosity reducing polycarboxylic water reducer prepared by the invention is low in dosage and high in water reducer rate, the air entraining property of the low air-entraining viscosity reducing polycarboxylic water reducer can be reduced, the strength and durability of the concrete are improved while the viscosity of high grade concrete is remarkably reduced, and the raw materials are low in cost, the reaction condition is mild, and the low air-entraining viscosity reducing polycarboxylic water reducer is suitable for being popularized and applied.