The invention relates to a water reducer used for concrete and a preparation method thereof, in particular to a polycarboxylate water reducer and a preparation method thereof, belonging to the field of concrete admixture technology. The polycarboxylate water reducer comprises the following components, based on parts by mass: 100 parts of polyoxyethylene methyl alkenyl ether macromer, 4-18 parts of unsaturated carboxylic acid and acid amide or acid anhydride, 1-6 parts of unsaturated sulfonate monomer, 0.5-3 parts of oxidizer, 0.01-0.5 parts of a reducing agent, 3- 15 parts of sodium hydroxide and 100-200 parts of water. According to the method, the characteristics such as excellent ability of water-reducing, low slump loss and good adaptability to different types of cement and concrete admixture are shown in the product at low accretion by means of one time addition; and the cost performance is high. The method has advantages that requirements for production equipment and production conditions in industrial production can be further lowered, energy consumption is reduced, and the method is economical, environment-friendly and beneficial to be generalized.