The fluorescence colors of oil inclusions can be influenced by many geological factors including thermal maturity. However, the ultraviolet (UV) irradiation during laboratory process can also induce a significant alteration in fluorescence spectra of oil inclusions, which is seldom considered when using fluorescence to classify oil inclusions. Four oil inclusions from the Maokou Formation in the central Sichuan Basin, China were performed continuous UV irradiation. Their fluorescence spectra were recorded over time. The vapor / liquid phase ratio was measured by a confocal laser scanning microscope (CLSM). The results reveal a generally irreversible alteration of oil inclusion fluorescence spectra. λ_max shows long-time stable and instant mutation, whereas QF-535 and Q_650/500 exhibit monotone increasing trend. Recorded λ_max can represent the original based on spectral shape analysis, however, recorded QF-535 and Q_650/500 require a regression model to predict original values. Besides, there is a potential change in vapor / liquid ratio of oil inclusion after long and intense UV irradiation. The conclusion is that the thermal maturity determination by fluorescence spectra of oil inclusions should be used with care, because the spectra are easily altered during petrography study before being recorded.