To reveal the effect of clay on the dispersibility of polycarboxylate superplasticizer(PC), the fluidity and viscosity of cement paste are chosen as performance indexes to study the effect of clay on the rheological properties of cement paste mixed with PC. The influence of filtrate of clay on molecular structure and the regularity of adsorption of PC on the surface of clay particles in alkaline environment are also analyzed by IR and UV. The results show that PC has no dispersing effect on the cement paste when clay content reaches to 15%. Increasing the dosage of PC can reduce the negative impact of clay. The filtrate of clay can’t change the molecular structure of PC and doesn’t affect the dispersibility of PC. In simulated alkaline environment (saturated solution of calcium hydroxide), the adsorption speed of clay on PC is rapid, whose adsorption capacity of clay on PC is about 4 times of cement’s, and clay will have been reached its equilibrium adsorption amount in the initial period (within 6 min).