As an important hormone, melatonin has a wide range of functions in human body. Inappropriate light at night (LAN) will cause abnormal human circadian rhythm and then lead to melatonin secretion suppression. Melatonin suppression depends on correlated color temperature (CCT) and wavelengths of LAN. So far there was no quantitative algorithm for melatonin suppression by LAN. A relative spectral sensitivity curve of human plasma melatonin suppression was fitted and an algorithm was presented for predicting melatonin suppression by LAN. Theoretical basis and a method were provided for instructing safety strategy of architectural light at night. Also, the results could be applied in controlling light pollution, formulating the standard of safety nocturnal environment illumination and some other fields.