Well-dispersed citrate-capped silver nanoparticles were synthesized in aqueous solution by reducing silver(Ⅰ) nitrate with sodium borohydride.As a nucleophilic agent，iodide ion could be easily adsorbed on the surface of silver nanoparticles through donating its electron pair on the unoccupied orbital of silver nanoparticles.Due to the low solubility constant(Ksp=8.52×10-17) of AgI，the addition of silver(Ⅰ) ion could induce the aggregation and color change of silver nanoparticles by forming AgI on the surface of iodide ion adsorbed silver nanoparticles.Based on this phenomenon，a highly sensitive colorimetric detection of silver(Ⅰ) ion was developed.Silver(Ⅰ) ion could be detected at 2.0 nmol/L by naked-eye.The linear range was 1.5-1 000.0 nmol/L，and the detection limit was 0.8 nmol/L when silver(Ⅰ) ion was detected by absorption spectrometric method.The common metal ions could not induce color change and absorption variation of iodide ion adsorbed silver nanoparticles even at the concentration of 2.0 μmol/L，indicating acceptable selectivity of the method to silver(Ⅰ) ion.High resolution transmission electron microscopy，dynamic light scattering and reason light scattering were used to investigate the detection mechanism of the system.All these characterization techniques confirmed the aggregation of silver nanoparticles when iodide ion and silver(Ⅰ) ion were added due to the loss of the negative charges of silver nanoparticles after the formation of neutral AgI.The developed method was used to detect silver(Ⅰ) ion in tap water and lake water by the standard addition methods with satisfactory results.