Objective To explore the disease burden and impacting factors of postoperative pneumonia (POP) in patients undergoing coronary artery bypass grafting (CABG), provide basis for the prevention and control of POP. Methods Clinical data of 5 566 patients who underwent CABG from January 1, 2020 to November 30, 2024 and met the study criteria were retrospectively collected. Independent risk factors for severe POP and postoperative 90-day mortality were analyzed, and differences in medical cost-related indicators among subgroups were compared. Results Among 5 566 CABG patients, 627 had POP,with an incidence of 11.3%, out of which 1.4% (n=76) were severe POP. Risk of postoperative 90-day mortality in POP patients was higher than non-POP patients (HR=4.16, 95%CI: 2.39-7.26). Univariate and multivariate Cox regression analysis showed that duration of surgery, red blood cell transfusion, multidrug-resistant organism (MDRO) infection, and mixed bacterial infection were associated with the occurrence of severe POP; Mixed bacterial infection and duration of controlled mechanical ventilation were independent risk factors for postoperative 90-day mortality. The median length of hospital stay, median length of intensive care unit (ICU) stay, and median hospitalization cost for POP patients were all higher than non-POP patients, with the increases of 25.0% (15.0 days vs 12.0 days), 100% (8.0 days vs 4.0 days), and 26.1% (183 911.6 Yuan vs 145 851.6 Yuan), respectively. Among POP patients, those with severe POP, mixed bacterial infection and MDRO infection further increased the length of hospital stay and cost: compared with the common POP group, the above three indicators in severe POP group increased by 63.3%, 83.3%, and 57.9%, respectively; Compared with non-mixed infection, the above three indicators in patients with mixed infection increase by 40.0%, 46.7%, and 57.7%, respectively; Compared with patients with sensitive bacterial infection and pathogen-negative, length of postoperative hospital stay in MDRO-infected patients both increased by 14.3%, length of ICU stay increased by 66.7% and 150.0% respectively, hospitalization cost increased by 11.7% and 21.5%, respectively; The above differences were all statistically significant (all P<0.001). Conclusion Duration of surgery, red blood cell transfusion, MDRO infection, and mixed bacterial infection are independent risk factors for severe POP after CABG, and mixed bacterial infection can further increase the risk of postoperative 90-day mortality. POP can significantly increase the consumption of medical resources, and its impacting extent exhibits heterogeneity depending on the severity of disease condition and characteristics of infection.