The monodisperse colloid silver with a 20 nm average diameter was prepared by UV-irradiation photochemical method. The kinetic characteristic on the reaction of Ag nanoparticles with cyanide was studied based on the surface plasma resonance (SPR) spectrum, and the mechanisms of molecule O2 participating in the reaction and the influence of OH- ions on the reaction were discussed. The coordination number of forming cyanocomplexes was also determined