Abstract: The effects of concentration of NaCl and pH values on the adsorption isotherms of poly（diallyldimethylammonium chloride）（ PDADMAC）on SiO2 microspheres and Zeta-potential of adsorbed-silica in different pH buffer have been investigated. The results show that the adsorption of the polyelectrolyte increases with increasing salt concentration and pH values，respectively. It follows the Langmiur isotherm. The isoelectric point of SiO2 microspheres gradually migrates toward the high pH values with increasing the adsorption of PDADMAC. Under alkaline conditions the PDADMAC adsorption on SiO2 microspheres shows good stability of disperse. The X-ray photoelectron spectroscopy（XPS）analysis of the PDADMAC adsorpted SiO2 microspheres sample exhibits the presence of nitrogen-containing intermediates with N1s XPS peaks at the binding energy of 401.7 eV. The O1s spectra show two distinct peaks which are oxygen-containing silica corresponding lower binding energy and adsorping oxygen corresponding higher binding energy，respectively. With the PDADMAC adsorption on SiO2 microspheres the peak of O1s contributed from the part of adsorping oxygen increases.