Objective To analyze the distribution of pathogenic bacteria and antimicrobial resistance of multidrug-resistant organisms (MDROs) in elderly patients with pulmonary infection (PI) after cardiothoracic surgery, and to explore the risk factors for postoperative MDRO PI. Methods Elderly patients with postoperative PI after cardiothoracic surgery in Shanghai Chest Hospital From January 2020 to December 2022 were selected as the study subjects. According to whether PI strains were MDROs, the patients were divided into the MDRO group and the non-MDRO group. The antimicrobial resistance rates of MDROs were calculated, and the risk factors for postoperative MDRO PI were analyzed. Results A total of 23 905 elderly patients underwent cardiothoracic surgery, and 289 cases developed postoperative PI, with an infection rate of 1.21%. Among them, 123 cases were classified as MDRO PI (MDRO group), and 166 cases were non-MDRO PI (non-MDRO group). Compared with the non-MDRO group, the proportion of patients with diabetes, mechanical ventilation duration >10 days, indwelling urinary catheter duration >10 days, indwelling central venous catheter duration >10 days, indwelling thoracic drainage tube >10 days, and use of ≥3 types of antimicrobial drugs was higher in the MDRO group, and the length of ICU stay was longer (all P < 0.05). Multivariate logistic regression analysis showed that diabetes, mechanical ventilation duration >10 days, and the use of ≥3 types of antimicrobial drugs were independent risk factors for postoperative MDRO PI in elderly patients who underwent cardiothoracic surgery (all P < 0.05). A total of 314 strains of pathogenic bacteria were isolated from sputum specimens of patients with postoperative PI, 137 of which were MDROs, mainly Pseudomonas aeruginosa (n=38, 27.73%) and Staphylococcus aureus (n=31, 22.63%). The resistance rates of multidrug-resistant Pseudomonas aeruginosa to imipenem, meropenem, levofloxacin, and ciprofloxacin were more than 68%. The resistance rates of multidrug-resistant Staphylococcus aureus to vancomycin, linezolid and rifampicin were 0. Conclusion Common pathogens causing postoperative MDRO PI in elderly patients underwent cardiothoracic surgery are Pseudomonas aeruginosa and Staphylococcus aureus. Diabetes, duration of mechanical ventilation, as well as type and quantity of antimicrobial use are closely related to the occurrence of MDRO PI. Targeted intervention measures should be taken to prevent the occurrence of postoperative MDRO PI.