Objective Community-acquired pneumonia (CAP) is a common infectious disease encountered in emergency departments, and timely severity stratification is essential for early risk identification and individualized treatment. Chest CT can accurately evaluate the extent of pulmonary involvement; however, its use for repeated bedside assessment is limited by radiation exposure, cost, limited portability, and the need for patient transport. In contrast, EIT is a radiation-free, noninvasive, and bedside functional imaging modality that can continuously monitor regional ventilation. Nevertheless, its relatively low spatial resolution limits its ability to directly characterize the anatomical extent of pulmonary lesions. Therefore, this study constructed a CT shape-prior-guided 3D CT-EIT composite image and explored its value for CAP severity assessment.Methods A total of 60 adult patients with CAP who were admitted to the Emergency Department of the Third Hospital of Hebei Medical University between October 2023 and October 2024 were retrospectively included. According to a five-lobe CT score ranging from 0 to 25, the patients were divided into a light pneumonia (LP) group and a moderate-to-severe pneumonia (MSP) group. Patients with CT scores of 0–8 were classified into the LP group (n=29), whereas those with CT scores of 9–25 were classified into the MSP group (n=31). Clinical indices, including oxygenation index (PaO₂/FiO₂), fibrinogen, and pneumonia severity index (PSI), were collected. The 3D EIT composite image based on the individual CT images of each patient was constructed, and the percentage of ventilation blocked regions (PVBR), ventilation distribution center error (VDCE), and global ventilation delay index (GVDI) were established. Receiver operating characteristic (ROC) curves were plotted to evaluate the discriminative performance, and Pearson correlation coefficient (r) was used to assess the correlation between 3D EIT ventilation indices and CT score.Results There were no statistically significant differences in baseline characteristics between the two groups (P>0.05). Compared with the LP group, the MSP group showed lower PaO₂/FiO₂ (166 mmHg vs. 298 mmHg, P<0.01), higher fibrinogen (4.59 g/L vs. 3.90 g/L, P<0.01), higher PSI (114.23 vs. 94.52, P<0.01), higher PVBR (39.33 vs. 26.33, P<0.001), higher VDCE (42.21 vs. 35.33, P<0.05), and higher GVDI (0.48 vs. 0.38, P<0.001). Among the conventional clinical indices, PaO₂/FiO₂ showed the highest area under the ROC curve (AUC=0.723). Among the 3D EIT ventilation indices, PVBR showed the best performance, with an AUC of 0.775 and a specificity of 96.6%. CT score was negatively correlated with PaO₂/FiO₂, but positively correlated with fibrinogen, PSI, and 3D EIT ventilation indices. Among these, CT score showed the strongest correlation with PVBR (r=0.696).Conclusion 3D CT-EIT composite imaging is feasible for rapid bedside functional assessment of CAP severity. PVBR, VDCE, and GVDI provide complementary information on the extent of ventilation blockage, the spatial redistribution of ventilation, and the temporal delay in ventilation, respectively. Among these indices, PVBR showed the best discriminatory performance and the strongest correlation with CT-defined lesion severity. Although 3D EIT cannot replace CT for structural diagnosis, it may serve as a practical radiation-free functional complement for bedside severity stratification, serial ventilation monitoring, and early clinical decision-making in patients with CAP.