In this article, the authors studied the effect of Cl^– on the fluorescence of methyl orange solution containing silver nanoparticles by UV–visible absorption spectra, fluorescence spectra, and transmission electron microscopy. The results indicated that there is little change in the UV–visible absorption spectra of methyl orange after adding Cl^– into the mixed solutions of the methyl orange and silver colloid. With the increase in Cl^– content, the fluorescence of S₁->S₀ decreases first and then increases, and the fluorescence of S₂->S₀ increases, which almost has nothing to do with the pH value of methyl orange solution. The fluorescence enhancement ratio of S₁->S₀ in the solution of pH = 6.1 was higher than that in the solution of pH = 2.1. The authors discussed the mechanism of the effect of Cl^– on the fluorescence of methyl orange solution containing silver nanoparticles in terms of interaction and energy transfer. The results indicated that the effect of Cl^– on the fluorescence of methyl orange solution containing silver nanoparticles depends on the bridge role and the competitive adsorption between nanoparticles and methyl orange molecules, molecular adsorption mode, and so on.