The basic raw materials and synthesis method for polycarboxylic acid type water-reducer are introduced, the working mechanism, the changes of molecular structure and property are also discussed. A molecular structure model of water-reducer is then proposed. The best properties of a polycarboxylic acid type water-reducer may be obtained if the amphoteric and non-ionic groups are all in the same molecular structure. The conclusion can be applied for the design of water-reducer with high performance.