Objective To explore the predictive value of the ratio of serum procalcitonin (PCT) to Simpson's diversity index (SDI) in alveolar lavage fluid for the short-term prognosis of patients with bacterial pneumonia in intensive care unit (ICU). Methods Medical records of 56 patients with bacterial pneumonia who were performed alveolar lavage fluid metagenomic next-generation sequencing (mNGS) technology in ICU of the Affiliated Hospital of Yangzhou University from October 2019 to July 2021 were retrospectively investigated, according to the acute physiology and chronic health evaluation (APACHE-Ⅱ) scoring system within 24 hours after admission to ICU, patients were divided into non-critical group (n=21) and critical group (n=35). Taking death caused by bacterial pneumonia as the end event, 28-day prognosis was recorded, patients were divided into survival group (n=38) and death group (n=18). SDI, PCT, C-reactive protein (CRP), PCT/SDI and CRP/SDI of patients in each group were compared and analyzed. Results Compared with non-critical group, critical group had higher levels of PCT/SDI and PCT, longer duration of ventilator-assisted ventilation, and higher 28-day mortality (all P < 0.05); compared with survivor group, death group had lower SDI, as well as higher PCT/SDI and PCT (both P < 0.05); SDI was negatively correlated with duration of ventilator-assisted ventilation (r values were -0.655, P < 0.001). PCT, PCT/SDI were positively correlated with duration of ventilator-assisted ventilation time (r values were 0.660, 0.734, respectively, both P < 0.001). The receiver operating characteristic curve (ROC curve) shows that the area under the ROC curve (AUC) of PCT/SDI for predicting 28-day mortality of patients was 0.851, followed by PCT+SDI (0.845), PCT (0.808), SDI (0.785), and CRP/SDI (0.731), when optimal cut-off value of PCT/SDI was 11.56, sensitivity and specificity for predicting 28-day mortality of patients were 89.5% and 66.7% respectively. Cox regression analysis showed that high PCT/SDI value (HR=1.562, 95%CI: 1.271-1.920, P=0.031) and high PCT level (HR=1.106, 95%CI: 1.021-1.198, P=0.024) were independent risk factors for death of ICU patients with bacterial pneumonia. Conclusion PCT/SDI, PCT, PCT+SDI, SDI and CRP/SDI can all be used as short-term prognostic indicators for bacterial pneumonia in ICU patients. Compared with other indicators, PCT/SDI is more valuable in predicting the short-term prognosis of patients.