The surface plasmon resonance (SPR) of noble metallic nanostructures has attracted considerable attention due to its wide application. It plays a dual roles in catalyzing special surface reactions and inducing surface-enhanced Raman spectroscopy (SERS) effect. For the latter, the surface Raman signal is enhanced dramatically. It allows monitoring the SPR catalysis reaction by SERS at nanostructures with extremely high sensitivity. However, the SERS investigation of SPR catalysis reaction is still significantly restricted to the N=N coupling. The extension on the reaction type and improvement in the catalytic activity and efficiency are highly desired. Herein, the SERS technique is employed to investigate the decarboxylation reaction of Ortho-mercaptobenzoic acid (OMBA) adsorbed on Au nanoparticles monolayer film (Au MLF). The self-assembly fabricated the Au MLF at air/liquid interface, and it exhibited uniform distribution of “hot spots”. By using as substrate, the influence of solution pH, laser power and irradiation duration on the surface decarboxylation reaction was explored accordingly. The results demonstrated that the decarboxylation reaction of OMBA was occurred in the neutral or alkaline solution at Au MLF, and the product was thiophenol (TP). It was absent in the acidic solution and the activity of decarboxylation reaction activity in the alkaline solution was higher than that in the neutral solution. The stronger laser power brought the higher activity of decarboxylation. It indicated the linear relationship between the SERS intensities and laser irradiation duration, and the efficiency of SPR catalyzed decarboxylation was improved by prolonging the irradiation duration. The preliminary results are beneficial for extending the SPR catalyzed surface reaction and understanding of the surface reaction mechanisms.