The dual-wavelength spectrephotometry and para-dimethylaminobenzaldehyde chmmogenic reaction were used to investigate the tryptophan content in silk fibroin of the inner

middle and outer cocoon layers and the distribution of tryptophan along the radial direction of silk fibers．The tryptophan content changes after light exposure and dry heating treatments were also examined．The results show that there ale obvious differences in tryptophan content of fibroin in different cocoon layers The order of tryptophan content is as follows：outer cocoon layer> middle cocoon layer > inner cocoon layer．The distribution of tryptophan in silk fiber is not even．with the highest tryptophan content in the outermost surface layer

and the lowest tryptophan content in the inner layer；the content in the middle layer is higher than that in the hypo-outer surface．Lilght exposure and dry heating treatment destroy tryptophan and reduce tryptophan content in fibroin．