The invention discloses a polycarboxylate superplasticizer and a preparation method thereof. The polycarboxylate superplasticizer has a pH value of 6.8 to 7.2 and a solid content of 38 to 43% and is prepared from the following components by weight: 60 to 110 parts of methyl allyl polyethenoxy ether, 40 to 90 parts of isopentenyl polyoxyethylene ether, 20 to 50 parts of allyl alcohol polyoxyethylene ether, 110 to 210 parts of unsaturated carboxylic acid or derivatives thereof, 20 to 50 parts of maleic anhydride, 0.5 to 5.6 parts of an oxidizing agent, 0.3 to 1.2 parts of a reducing agent, 0.4 to 1.9 parts of a chain transferring agent, 360 to 746 parts of water and a proper amount of a sodium hydroxide solution with a concentration of 48%. The invention provides the method for preparing the polycarboxylate superplasticizer from polyether macromonomers of three structures and the method is capable of effectively improving the comprehensive performance of the polycarboxylate superplasticizer and easy to industrialize; and the obtained polycarboxylate superplasticizer has high water reducing performance and high slump loss resistance.