Understanding the molecular orientation is crucial for revealing the mechanism of chemical reactions. However, how to accurately monitor the molecular orientation remains a great challenge because of the motion of the molecules during the chemical reaction. Here, surface-enhanced Raman spectroscopy (SERS) was used to monitor the molecular reorientation from the plasmon-catalyzed conversion of p-aminothiophenol (PATP) to p,p'-dimercaptoazobenzene (DMAB). Spectral change due to molecular reorientation, which is often overlooked, is clearly extracted through the protonation of the PATP molecules to suppress the conversion of PATP to DMAB. Meanwhile, by monitoring the Raman features, the reaction process and molecular reorientation are simultaneously recorded, offering fresh insights on the long-standing puzzle that why the conversion to DMAB is suppressed with concentrated PATP: this is because the over-crowded PATP adsorbed on the substrate hampered its own space-taking reorientation (similar to the space-steric effect) which is a prerequisite for producing DMAB.