Polycarboxylate macromolecule was synthesized using acrylic acid, methacrylate, sodium methallyl sulfonate and allyl polyoxyethylene ether，with ammonium persulphate used as initiator for radical aqueous solution polymerization. The polycarboxylate prolonged the gypsum plaster’s setting time. The plasters containing 0.2% polycarboxylate presented a increase of bending strength and compressive strength of 31.1% and 32.6% in 24 h, respectively. The water absorption rate of the modified plaster was 9.1% lower than that of pure plaster.