Abstract: The photodegradation of drugs under natural light and various lamp lightirradiation was studied and the shelf-life of drugs under indoor natural light irradiation was predictedwith hydroxyprogesterone caproate injection as an example.The equivalent influences of differentlight sources on the photostability of the above drug were obtained.Therefore,the natural lightphotostability and the shelf-life of drugs under indoor natural light irradiation can be studied with lamplight exposure experiment.The photodegradation of hydroxyprogesterone caproate injection obeys zero order kinetics :C=C₀-kEt,and its shelf-life under indoor natural light irradiation is about 1.9 years.