This study aims to evaluate the efficacy and safety of prophylactic oral N-acetylcysteine (NAC) for facilitating platelet engraftment in patients with severe aplastic anemia (SAA) receiving haploidentical hematopoietic stem cell transplantation (haplo-HSCT). This is a prospective, multicenter, randomized controlled trial enrolling a total of 142 patients with SAA scheduled for their first haplo-HSCT, who will be randomly assigned at a 1:1 ratio to the NAC prophylaxis group or the control group, with 71 subjects in each arm. Patients in the intervention group will receive oral NAC 400 mg three times daily from Day -14 before transplantation to Day +60 post-transplant, while the control group will receive no prophylactic NAC, with all other transplant-related treatments identical between the two groups. The primary endpoint is the cumulative platelet engraftment rate by 2 months after transplantation. Secondary endpoints cover neutrophil engraftment rate, incidence of poor hematopoietic reconstitution, cumulative blood product transfusion volume, graft-versus-host disease (GVHD), overall survival, GVHD-free and failure-free survival, and biomarkers reflecting bone marrow hematopoietic microenvironment reconstruction. Safety outcomes will be assessed via adverse events graded according to the National Cancer Institute Common Terminology Criteria for Adverse Events (NCI CTCAE), version 5.0. Statistical analyses will be performed using R 4.4.0 software, primarily adopting competing risk models and the Kaplan-Meier method based on full analysis set, per-protocol set and safety set. This trial will clarify intergroup differences in platelet recovery, other efficacy endpoints and safety profiles, verify the clinical benefits and safety of NAC, and generate high-quality clinical evidence for prophylactic intervention targeting platelet engraftment after haplo-HSCT in SAA patients to optimize clinical management strategies.