The influence of light on materials is inevitable and irreversible, and the photo-induced damage has an accumulated effect. The International Council of Museums classifies materials into four classes based on photochemical stability: insensitive, low sensitive, relatively sensitive, and extremely sensitive. Different photosensitive materials have different capacities to withstand irradiation. Therefore, it is necessary to study the influence of illumination on pigments, which has a guiding significance for protecting and restoring cultural relics. Realgar is a well-known photosensitive material. Under visible light irradiation, realgar can degenerateinto uzonite (As4S5) and arsenolite (As2O3), and uzonite as an intermediate product is desulfurized to generate pararealgar. The photoinduced degradation is very common, but the results and time of degradation of realgar vary with the irradiation conditions. Raman spectroscopy is a non-destructive testing technology which can be used for qualitative and semi-quantitative analysis of material components. In the photoinduced degradation of realgar, the changesinmaterial structure and composition can be judged by the changesinRaman peak position and intensity. The WLED is commonly used for internal lighting of ancient buildings. According to the time when the Raman signal of realgar completely disappeared under different irradiance measured by Raman technology, the relationship between irradiance y and realgar degradation time x was y=e9.057/x0.861, indicating that the time needed for complete degradation of realgar was shorter with higher irradiance. In addition, the study also found that under WLED irradiation, the final products of realgar were the coexistence of pararealgar and uzonite. Itis because, under white light irradiation, pararealgar,as the degradation product of realgar, degraded into the intermediate uzonite under the irradiation of blue light contained in the white light source. In the internal lighting of ancient buildings, the irradiance received by the surface of the photosensitive pigment should be reduced as much as possible. Different lighting sources have different effects on realgar, and the relationship between the complete degradation time of realgar and irradiance is also different, but the lighting source used in this experiment is more consistent with the current lighting situation. Moreover, realgar is an important pigment in ancient buildings colorful paintings and murals. According to the relationship, ancient buildings can be repaired regularly in combination with the irradiance of the internal lighting of ancient buildings.