The adsorption rate, water absorption, volume swelling capacity of 4 kinds of clay, and the interlayer spacing of montmorillonite were tested. The adsorption capacity and mode of polycarboxylate superplasticizer(PCE) on different clays were studied. The volume change of solid and liquid phase in fresh concrete before and after adsorbing PCE by clay was calculated. The inhibition mechanism of clay on applying performance of PCE was explored. The results show that the adsorption of PCE on clay is higher than that on cement, and adsorption capacity of montmorillonite is the strongest of all. The side chain of PCE molecular is adsorbed into interlayers of montmorillonite, which increases the interlayer spacing(d(001)) of montmorillonite from 1443nm to 1863nm. Clay adsorbs more PCE and thus reduces the effective content for dispersing action. The volume of clay expands after absorbing water, increasing the solid phase volume and decreasing the liquid phase volume, and finally worsening the workability of fresh concrete.