The citric acid (CA) was added to the silk fibroin and Polyvinyl Alcohol (PVA) copolymer system. The blending 4% PVA was for improving the mechanical properties of the silk fibroin films. Citric acid amount varies from 0.5% to 2% of weight. The antithrombogenecity was characterized by in vitro clot time of prothrombin time (PT), the Activated partial thromboplastin time (APTT) and thrombin time (TT). The mechanical properties of the blending films were measured and the tensile strength and breaking elongation of films reached to 25.3MPa and 102%. The anticoagulant property results indicated that APTT and TT prolonged significantly, but PT has no effective change. We discussed the relationship between the structure and properties and proposed an anticoagulant mechanism.