Abstract: Ag nanoparticles were synthesized in a nanoreactor formed by an adsorption layer on the silica surface.The effect s of concent ration of water on the process of adsorption and reaction were analyzed. When the mass fraction of NaOH keeps constant , the equilibrium content of NaOH adsorbed on SiO₂ increases with the increase of concent ration of water , which changes f rom 0 to 1. 00 %. Two jump regions of equilibrium content are found f rom 0 to0. 05 % and f rom 0. 25 % to 0. 50 %. Analysis by XRD and TEM indicates that the agglomeration of Ag or Ag₂Oparticles becomes more moderate , it s dispersion on the surface SiO₂ becomes more uniform , and it s grain size becomes smaller with the increase of concent ration of water. The minimum grain size and uniform dispersion can beachieved when the concent ration of water is 0. 50 %. Based on the reduction mechanism of Ag + and essential principle of the adsorption process on the surface of SiO₂ , it can be concluded that the water-rich adsorption layer on thesurface of SiO₂ is formed gradually with the increase of water concent ration and the region of Ag generation changesfrom the surface of SiO₂ to the water-rich adsorption layer , where the concent ration of NaOH is so high that most ofAg are generated in it .