Objective To evaluate the ability of diaphragm excursion (DE) on admission in predicting the need for invasive mechanical ventilation (IMV) and prognosis of death in patients with severe pneumonia. Methods A retrospective study was conducted with severe pneumonia patients admitted to the Emergency Department of Beijing Chaoyang Hospital, Capital Medical University from September 2021 to August 2022. Diaphragm ultrasound were performed within 12 hours of admission to the emergency room. The patients were divided into mechanical ventilation (MV) group and non-mechanical ventilation (NMV) group according to the line whether IMV treatment was needed during hospitalization or not. And they were divided into death group and survival group according to the outcome within 30 days after admission. DE were compared between MV group and NMV group, then between death group and survival group. The predictive values of DE for the need of IMV and prognosis of death in severe pneumonia patients were further evaluated by receiver operating characteristic (ROC) curve. Results DE in MV group were lower than that in NMV group, the difference was statistically significant [1.15±0.60(cm) vs. 1.67±0.79(cm), P＜0.05]. DE in death group were lower than that in survival group, the differences was statistically significant [1.07±0.59(cm) vs. 1.91±0.70(cm), P＜0.01]. ROC curve analysis showed that the cut-off value of DE for predicting the need of IMV in severe pneumonia patients was 1.28cm (P < 0.05), the sensitivity was 68.8%, the specificity was 67.7%, and the AUC was 0.694 (95%CI: 0.547-0.841). The cut-off value of DE for predicting the prognosis of death in severe pneumonia patients was 1.21cm (P < 0.01), the sensitivity was 68.0%, the specificity was 82.1%, and the AUC was 0.816 (95%CI: 0.702-0.931). Conclusions DE had a certain early value in predicting the need for IMV and prognosis of death in severe pneumonia patients.