A method for inhibiting coagulation including administering to a patient in need thereof a pharmaceutical composition containing 21-(S)-Argatroban and/or a pharmaceutically acceptable salt thereof. Compared with 21-(R)-Argatroban, 21-(S)-Argatroban significantly prolongs coagulation time of whole blood (CT), recalcification time (RT), kaolin partial thromboplastin time (APTT), pro-time prothrombin time (PT), thrombin time (TT), and reduces platelet adhesion rate and platelet aggregation rate in healthy dogs. Therefore, 21-(S)-Argatroban has a stronger effect for inhibiting coagulation and reducing therapeutically effective dose and is suitable for use in a method for treatment and/or prevention of thrombosis and inhibiting platelet aggregation.