In this article, by using zeta (z) potential analyzer, X-ray photo spectroscopy, and UV-visible adsorption spectrophotometer, the adsorption characteristics and surface electrochemical properties of carboxymethyl starch (CMS) as water-reducing agent on cement particles and its effects on the fluidity of cement paste were studied in comparison with traditional naphthalene sulfonic acid-formaldehyde condensates, whose dispersion ability mainly depends on electrostatic repulsive force caused by z-potential. The results indicate that the adsorption conformation of CMS is dendritic, generating strong steric hindrance by branched chains and polar side chains of adsorption layer, and the dispersion capacity of CMS mainly comes from steric hindrance effect rather than electrostatic repulsive force.