By means of researching on chemical shrinkage, heat of hydration and electrical properties of the initial structure, and using the XRD, TG DSC, the retarding mechanism of KH polycarboxylic acid type water reducing agent has been studied. The results show that: KH polycarboxylic acid type water reducing agent can control the chemical shrinkage, resulting in the reducing by 20% of the chemical shrinkage of 1 d, and over 30% of the chemical shrinkage of 28 d; it also has strongly retarding ability, controlling the heat of hydration and deferring the structure to establish; the groups of -OH, -COO-,-SO3-,-OCH2CH2-, have adsorbing, complexing, electrostatic repulsion and steric hindrance abilities to control the hydration of the C3A and C3S, but doesnt effect the development of the structure.