The haplotype HSCT system including Bu(0.8mg/kg Q6hx2d)CTX(50mg/kgx4d)rATG(2.5mg/kgx4d) , established in Institute of Hematology of Peking University ,has been evaluated to be effective for acquired SAA.But some patients with high risk factors may not tolerate CTX 200mg/kg,alternative conditioning regimen including Bu/Fludarabine/dercreased CTX was studied in this trial.

Patients enrolled in this study would receive Bu (IV)0.8mg/kg Q6hx2d,Fludarabine 30mg/m2x5d ,CTX(cyclophosphamide) 25mg/kg/dx4d,rATG (thymoglobulin,Sang Stat,France) 2.5mg/kg/dx4d.

BM or Blood samples from patients were obtained to assess engraftment and chimerism after HSCT. The time point that investigators monitor BM or blood samples at 1 month,2 months, 3 months,6 months, 9 months and 1year,2years,3years 5 years after HSCT.

A：inclusion criteria

1. Patients diagnosed as acquired severe aplastic anemia(SAA) /very vSAA
2. patients with age 3-55 years
3. patients have no matched sibling donor
4. Patients have no matched unrelated donor
5. patients have no severe infection
6. Patients have no severe organ dysfunction
7. patients have risk factors of potential intolerance to previous condition regimen including BuCy(200mg/kg)and ATG
8. Consent form signed

B. Exclusion criteria ：

1. patients with congenital SAA/vSAA
2. patients with age< 3years or >55 years
3. patients with matched sibling donor
4. patients with matched URD
5. patients with severe infection
6. patients with severe organ dysfunction
7. pregnancy women
8. no Consent form signed