p-Mercaptoaniline (pMA, it is often called p-aminothiophenol (PATP)) can form self-assembled monolayer (SAM) through its thiol group on metal surfaces. It is also an important surface probe molecule in the surface-enhanced Raman spectroscopy (SERS). More recently, pMA has been used in molecular electronics as a single-molecule conductance wire to link two nanoscale metal leads, constructing a molecular wire junction. When pMA adsorbs on these metal nanostructurcs and nanogaps, there exhibit strongly detectable surface Raman signals of "non-totally symmetric" modes. The abnormal enhancement mechanisms proposed in the literature fall into three different categories, i.e., photoinduced charge transfer, tautomerization of benzenoid and quinonoid states, and the charge tunneling enhancement mechanism. Here, we present a novel mechanism to understand the observed SERS for pMA adsorbed on silver surfaces. On the basis of our theoretical calculations combined with the reports in literatures, we propose that the pMA molecules adsorbed on nanoscale rough surfaces of noble metals and nanoparticles undergo a catalytic coupling reaction to selectively produce a new surface species p,p'-dimercaptoazobenzene (DMAB), an aromatic azo compound, which is responsible for the experimentally observed Raman spectra. The present results are important for understanding many pMA-related experiments on noble nanoparticles. ; NSF of China [20433040, 10474082, 20573087]; National Basic Research Program of China [2007CB815303, 2009CB930703]