Abstract: The photostability of vitamin B₂, vitamin K₃, zinc sulfadiazine andvesnarinone, and the equivalent influences of different light sources on the photostability ofthe above drugs were studied under natural light and lamp lights. From this study, thephotostability of drugs under different light sources can be converted to each other, and thenatural light photostability of drugs can be conveniently,obtained from lamp light exposureexperiments. The relationship between the surface reflectance of the above drugs and the cumulativeillumination of different light sources can be formulized as R=A+B(Et) + C/( Et+ D)accurately, and the cumulative illumination of different light sources irradiated on theabove drugs can be converted to each other equivalently.