Disclosed is a method for preventing or treating thrombosis, the method comprising administering a composition containing a hemp (Cannabis sativa L.) leaf or flower extract as an active ingredient. The hemp (Cannabis sativa L.) leaf or flower extract inhibits thrombus formation-related enzymes and coagulation factors and exhibits potent antithrombotic activity through inhibitory activity against platelet aggregation which triggers blood clotting, but with no lytic activity on human erythrocytes. In addition to being stable to heat, the extract does not lose the inhibitory activity against coagulation factors and thrombus formation-related enzymes even in the acidic condition of pH 2 and the plasma environment. Therefore, the extract is expected to find applications in preventing and treating thrombosis such as ischemic stroke and hemorrhagic stroke through blood circulation improvement.