The invention relates to a high-efficiency water reducing agent for cement paste, and belongs to the technical field of a water reducing agent. The water reducing agent is prepared from 20 to 30 partsof polyether 220, 20 to 30 parts of maleic anhydride, 5 to 10 parts of sodium methallyl sulfonate, 3 to 5 parts of methacrylic acid, 3 to 5 parts of crylic acid, 0.3 to 0.5 part of ammonium persulfate, 0.3 to 0.5 part of sodium bicarbonate and 80 to 100 parts of deionized water. According to the invention, by monomers, such as the polyether, the maleic anhydride and the sodium methallyl sulfonate, under the action of an initiator, the polycarboxylic water reducing agent with an excellent dispersing effect can be prepared through ternary polymerization reaction. The high-efficiency water reducing agent has the advantages of small use quantity, high water reducing rate, wide application range and the like. A cement product prepared by adopting the water reducing agent has the performances of high intensity and high durability.