A 39-year-old woman who had been receiving steroid treatments for systemic lupus erythematosus (SLE) and antiphospholipid antibody syndrome (APS) had 11 emergency hospitalizations over 18 months for diffuse alveolar hemorrhage (DAH). She had severe mitral stenosis and regurgitation, so mitral valve replacement with a bioprosthetic valve was performed in November 2022. There was no recurrence of DAH until May 2023 (six months after the surgery). However, her mitral stenosis worsened, and DAH recurred following postoperative thrombotic valve formation. Inadequate postoperative antithrombotic therapy might have led to the formation of a thrombotic valve; however, we were able to improve the patient's mitral stenosis and thus prevent the recurrence of DAH through antithrombotic therapy reinforcement. In conclusion, we considered that not only SLE/APS-related immune-mediated capillaritis but also chronic increased pulmonary capillary pressure and left atrial pressure caused by mitral valve disease may have promoted DAH. Additionally, strict and long-term antithrombotic therapy is particularly important in patients with APS who undergo bioprosthetic valve replacement procedures.