Abstract: Pulmonary infections are the most common complications in immunocompromised host (ICH). The immune system of ICH was in inhibitory condition because of long-term use of immunosuppressive agents, such as glucocorticoids, antimetabolite medicine and alkylating agents. Therefore, compared with the immunocompetent host, the condition is more critically serious in ICH with more difficult diagnosis and treatment,and higher motality after pulmonary infections. Early identification of pathogens and early treatment are helpful for improving the prognosis and decreasing mortality.