Abstract: In order to improve the mechanical properties of electrospun silk fibroin (SF) ultrafine fibrous membranes, SF and poly(butylene succinate) (PBS) were dissolved in 1, 1, 1, 3, 3, 3hexafluoro2propanol (HFIP) to fabricate SF/PBS composite ultrafine fibrous membranes by electrospinning. After the composite ultrafine fibrous membranes prepared by using different mass ratios of SF to PBS were treated by methanol, the morphology, structure, and mechanical properties of membranes were characterized by means of FESEM, FTIR, XRD, DSC and Instron 5543 tensile tester respectively. The results show that the average diameter of composite ultrafine fibers increases from 289nm to 425nm and the crystallinity of composite ultrafine fibers increases with the adding of PBS. Especially the mechanical properties are improved. The tensile failure stress of composite ultrafine fibrous membranes first decreases and then increases with the adding of PBS, and the failure strain gradually increases. When the blend mass ratio is 50/50, failure stress is close to 16 MPa and failure strain is up to 50%. The morphology and mechanical properties of ultrafine fibrous membranes can be adjusted and controlled effectively by adding PBS.