This study aims to compare the efficacy and safety of HLA-haploidentical hematopoietic stem cell transplantation (HLA-haplo HSCT) versus optimal immunosuppressive therapy (IST) as first-line treatments for severe aplastic anemia (SAA) through a real-world cohort design. The selection of treatment regimens for subjects is based on clinical decision-making in real-world practice, comprehensively considering factors including patient age, donor matching status, comorbidities, and treatment preferences, with non-randomized group allocation.