Objectives: To describe the underlying diseases, microbiologic examination and severity of hospitalized patients with Pneumocystis jirovecii pneumonia (PJP) in a tertiary Chinese hospital. Methods: We conducted a retrospective analysis of 485 identified PJP patients who were admitted to our hospital between January 2013 and December 2021. Results: Among the 485 enrolled PJP cases, there were 237 males and 248 females, aging (53.3±16.2) years (range from 14 y to 88 y). They were divided into 8 subgroups with variable underlying diseases. There were 209 cases with connective tissue diseases(CTD), 27 cases with non-hematologic malignancies, 38 cases with hematologic malignancies, 81 cases with kidney diseases, 33 cases with idiopathic interstitial pneumonia(IIP), 30 cases infected with human immunodeficiency virus (HIV), and 42 cases with miscellaneous underlying diseases. In the CTD group, there was more females than males, while male patients were predominant in both the malignant and the HIV groups. The Pneumocystis was identified in 44.95%(218/485) sputum samples and 92.01%(265/288) bronchoscopic samples. Pneumocystis asci were observed at direct microscopic examination with Grocott's methenamine silver stain in 4.95%(24/485)sputum samples and 9.72%(28/288)bronchoscopic samples. Pneumocystis DNA fragments were identified by PCR analysis in 43.09%(209/485)sputum samples and 90.63%(261/288)bronchoscopic samples. Among the 8 groups, cytomegaviremia and respiratory failure were most common in the HIV-infected PJP group, but the rates of mechanic ventilation, intensive care unit (ICU) admission and death were the lowest. There were less PJP patients in the IIP group (IIP-PJP) who received mechanic ventilation and admitted to ICU than the other groups except HIV-infected PJP group. However, the mortality rate was highest for the IIP-PJP group. Conclusions: CTD was the most common predisposed underlying disease for our enrolled PJP cases. Cytomegaviremia and respiratory failure were common in HIV-infected PJP patients, but the prognosis of HIV-PJP was slightly better than the others. The disease was more severe, rapidly progressive and fatal in the IIP-PJP group.