The condition of sericin in ancient silk is one of the important basis for the selection of conservation treatment. The aim of this research is to understand how the aged silk fiber changed and the applicability of FTIR as an effective instrument for studying the deterioration of silk textile with different content of sericin. The results indicate that there indeed are differences among the spectra of silk fibre with different sericin contents, and silk fiber with different sericin has different characteristic bands in FTIR after light aging and heat aging. The ratio of 1445 cm-1/1625 cm-1 could be used to distinguish the content of sericin on silk.