In the mixing process of concrete, the addition of auxiliary cementitious building materials can improve the performance of concrete while saving resources, but due to the compatibility problem of raw materials, there shall occur poor fluidity and abnormal setting etc. for the early-period concrete. For this, based on the adsorption mechanism of water reducer on auxiliary cementitious building materials, this paper carries out the study on the adsorption characteristics of three kinds of polycarboxylate superplasticizer (PCS) on the surface of fly ash, and solves the problem of relationship between the PC dosage/adsorption capacity and particle size of fly ash. The results show that the PC structure has a significant influence on the adsorption capacity, and its adsorption capacity on the surface of fly ash gradually increases and becomes saturated with the increase of its dosage. This study is of great significance for the selection of water reducer and the dosage application.