Raman spectroscopy, as an important spectroscopic tool, can be used for structural characterization of molecules as well as qualitative and quantitative analysis due to high detection sensitivity. Sulfonamides (para-amino-benzenesulfonamide) (PABS) are widely used as a large class of antibiotics. It was noted that there were significant differences between normal Raman spectrum and the SERS spectrum of sulfanilamide in alkaline solutions. Our density functional theory (DFT) calculations provide complete vibrational assignments. The further theoretical results show that for adsorbed sulfonamides(PABS) surface catalytic coupling reaction occurs. The product azo compound is responsible for the observed SERS spectra of adsorbed on silver surfaces.