Acquired hemophilia A is a very rare but life-threatening disorder caused by autoantibody against coagulation factor VIII. The incidence was much rarer in young people. In this case report, a young woman presented with spontaneous muscle hematoma. Because of pain and limited range of motion, she underwent surgery for resolution at first. However, her symptoms and hemorrhage worsened. She was diagnosed with acquired hemophilia A. We started combination treatment with bypassing agent, activated prothrombin complex and immunosuppressants immediately and the results were successful. The acute bleeding was controlled and autoantibody was completely resolved.