The influence of adding KCI to silver colloid on light absorption of the colloid was studied and compared with that of adding pyridine to the colloid. It was found that the light absorption was increased due to the formation of an atomic cluster layer on the Ag particle surfaces caused by adding KCI, and that the original surfaces plasmon resonance absorption was reduced. At the same time, the linear-chain aggregation caused by adding KCI lead to an apparent red shift of the plasmon resonance absorption, which sometimes was difficult to observe. The influence of these changes on SERS was also discussed.