The optical enhancement effect and catalytic activities produced by surface plasmon resonance (SPR) of metallic nanostructures have become one of the hot fields in surface scientific research. The SPR and electrochemical control combine to induce and catalyze the unconventional reactions, and electrolyte solutions with different pH values affect the SPR photocatalytic reaction by changing the adsorption form of surface adsorbed molecules. In this study, the adsorption and reaction behaviors of the isomers of hydroxythiophenol were served as probes modified on the Ag electrode and were investigated by the combination of electrochemistry and surface-enhanced Raman spectroscopy (SERS). The results revealed that the SPR-catalyzed dehydroxylation reaction of hydroxythiophenol with different hydroxyl substituent positions exhibited different sensitivity to the pH value of the solution. The C—O bond peak intensity of o-hydroxythiophenol (OHTP) was related to the pH value of the solution. The O end was easier to interact with metal and adsorb on the surface, and it improved with increased pH. Under alkaline conditions, the dehydroxylation reaction of p-hydroxythiophenol (PHTP) was completely inhibited, and it could occur in both meta-hydroxythiophenol (MHTP) and OHTP. MHTP held the highest SPR catalytic dehydroxylation reaction efficiency in neutral (pH 7) solution, which was about 1.36 times that of acidic (pH 2) and 2.70 times that of alkaline (pH 12). OHTP exhibits the highest reaction efficiency in alkaline (pH 12) solution, which was about 13.71 times that of acidic (pH 2) and 4.95 times that of neutral (pH 7). SPR-catalyzed dehydroxylation was mainly contributed by two approaches non-deprotonation conditions and Ag—O formation. The dehydroxylation reaction of MHTP and OHTP under acidic conditions was mainly due to the undeprotonated hydroxyl reaction, and the formation of Ag—O mainly caused the alkaline conditions after deprotonation. Under neutral conditions, both contributions occurred simultaneously. For MHTP, due to the steric hindrance, only part of the molecules was deprotonated to form Ag—O, which promoted the catalytic dehydroxylation of SPR. Therefore, the simultaneous action of the two effects in the pH 7 solution led to the highest catalytic efficiency. For OHTP molecules, the O terminal in the deprotonated state was more likely to interact with the electrode surface, and the degree of deprotonation of the hydroxyl group as the pH increases was more thorough, more conducive to the dehydroxylation reaction. The dehydroxylation reaction in the pH 12 solution mainly occurred in Ag—O, where the efficiency was the highest. The study of the isomer structure and the pH of the medium on the SPR-catalyzed dehydroxylation reaction was of great significance for broadening the types of SPR catalytic reactions and analyzing the mechanism at the molecular level.