To investigate the clinical features and outcome of severe pneumonia after allogeneic hematopoietic stem cell transplantation for malignant hematological disease, the clinical features of severe pneumonia, including symptoms, signs, blood-gas analysis and thoracic X-ray change were observed, the clinical therapeutical outcome is also evaluated. The results showed that after treatment with antibiotics or antibiotics + antiviral drugs or antibiotics + antiviral drugs + glucocorticosteroids, as well as mechanical ventilation, pulmonary changes in 17 patients with sever pneumonia completely disappeared, 1 case died. In conclusion, early diagnosis and proper treatment for complicated pneumonia after allogeneic hematopoietic stem cell transplantation can decrease the mortality.