Taking　the　fingerprint　characteristics　of　Raman　spectroscopy,　the　dimerization　of　p-nitrothiophenol　(PNTP)　to　4-nitrophenyl　disulfide　in　air　was　investigated,　which　could　be　accelerated　by　the　external　laser　illumination.　The　dimerization　was　qualitatively　confirmed　by　both　mass　spectroscopy　experimentally　and　by　Raman　spectrum　simulated　calculations　theoretically　and　quantitatively　analyzed　by　Raman　spectroscopy　in　a　simple　and　quick　manner.　On　the　basis　of　the　inductively　coupled　plasma　optical　mass　spectrometry　analysis,　the　dimerization　was　possibly　ignited　by　the　trace　metal　residues　as　catalyst　in　the　PNTP.　This　study　clearly　shows　that　it　is　very　important　to　strictly　and　preciously　control　the　experimental　conditions　of　Raman　measurement　and　other　laser-based　spectroscopies　to　avoid　any　wrong　conclusion　during　the　characterization　or　identification.　Different　to　the　plasmon-induced　photochemical　reactions,　we　reported　the　photochemical　reaction　of　PNTP　to　4-nitrophenyl　disulfide,　which　is　free　of　the　plasmoic　effect　from　the　metal　nanostrucutres.　On　the　other　hand,　the　photochemical　dimerization　reaction　may　provide　a　new　and　important　guideline　for　the　formation　of　thiols　to　disulfides.