Ground granulated blast furnace slag (GGBS) is the main by-product of steel industry and has been widely utilized as mineral addition in concrete. Due to the difference of chemical composition and hydration reactivity of slag and cement particles, the ionic environment of cement paste changed a lot and influenced the dispersion performance of PCEs. For clarifying the ionic environment change and their effects on the dispersing ability of PCEs. The ICP-OES was used for clarifying ionic environment of cement paste, and their effects on dispersing performance of PCEs were determined by flowability and rheological experiments. Test results showed that ionic environment influenced the surface charge of slag, affected adsorption behavior and conformation of PCEs inducing the different dispersing properties. PCE with longer side chain length or smaller acid-ether ratio exhibited better dispersion performance.