As a biodegradable polymer material, polylactic acid (PLA) has always been the focus of research on environmentally friendly materials. Polylactic acid itself has the disadvantages of poor heat resistance and toughness, which limits its application in electrical fields. Therefore, research on the modification of polylactic acid is very important for its application in more fields. Silk fiber (SF) is also a natural biodegradable biological material with good mechanical properties and insulation properties and is an ideal material for polylactic acid modification. The mechanical properties of silk fibers are mainly provided by fibroin protein, so it is usually necessary to perform surface treatment—degumming before use, but there may be sericin residue during degumming. Therefore, the influence of silk degumming rate on thermal, mechanical and insulating properties of polylactic acid composite was studied in this paper. Through simulation and experimental studies, it is found that the degumming rate has little effect on the glass transition temperature. The Young's modulus, bulk modulus and shear modulus of the material decreased with the decrease of the degumming rate, and the impact strength increased with the increase of the degumming rate. After adding silk fiber, the band gap of the material is obviously increased. This paper provides some reference for the application of silk polylactic acid composite in more fields such as electricity.