The abnormal changes in SERS of salicylic acid adsorbed on colloidal silver caused by adding KC1 are reported in this article. It was found that the SERS of some vibrational modes corresponsing to certain structures in salicylic acid were reduced rapidly. However, only the SERS intensity of v(-OH) vibration near 3436cm-1 incresed further because of KC1 addition. Also the influence of adding KC1 on SERS signals of a series of molecules including -COOH structure such as p-aminobenzoic acid, benzoic acid etc., were investigated. It was shown that Cl- could obviously weaken the SERS intensity of those molecules adsorbed on silver colloid. Both the phenomena have been discussed, and were compared with the case of pyriline. The results of transmission electron micriscopy observation and UV-visible spectral. experiments showed the agreement with the above conclusion.