Abstract: The differences in morphological structure,chemical constitution and micro\|structure of silk fiber in different cocoon layers were studied using a range of analytical techniques,scanning electron microscope(SEM),Fourier transform infra\|red spectroscopy in ATR mode (FTIR\|ATR),differential scanning calorimetry (DSC),amino acid analysis;The degumming loss percentage and the dyeing behavior with reactive dyes of the silk fibers in different cocoon layers were investigated.The results show that there are obvious differences in radial size and degumming loss percentage of silk fibers in different cocoon layers.Amino acid composition of silk fibroin in different cocoon layer is identical in category but different in content.There are some differences in micro\|structure and thermal property of silk fiber among different cocoon layers.