Objective To explore the influence of chlorhexidine mouthwash before surgery on postoperative pulmonary recovery and the occurrence of postoperative pulmonary complications (PPCs) in elderly patients. Methods A total of 174 elderly patients undergoing elective surgery under general anesthesia with tracheal intubation were enrolled. A stratified block randomization method was used, with stratification based on surgical type (total hysterectomy, partial colorectal resection, or hip arthroplasty). Within each stratum, patients were randomized in a 1:1 ratio with a block size of 4 to either the chlorhexidine group (CH group) or the normal saline group (NS group). Patients in the CH group rinsed with 15 ml of compound chlorhexidine mouthwash, while those in the NS group used an equal volume of normal saline. Mouth rinsing was performed under nurse supervision on the night before surgery, the morning of the surgery, and again after entering the operating room. The primary outcome was the lung ultrasound score (LUS) on postoperative day 1 (D1). Secondary outcomes included the LUS on postoperative day 3 (D3), and the incidence of postoperative pulmonary complications (PPCs), including pneumonia, fever, cough, and sputum production. Arterial blood gas analysis was performed at 5 minutes after tracheal intubation (T1), upon admission to the post-anesthesia care unit (T2), and at discharge from PACU (T3) to calculate the oxygenation index (OI), alveolar-arterial oxygen gradient (A-aDO2), respiratory index (RI), and dynamic lung compliance (Cdyn). Baseline patient characteristics, preoperative oral frailty score, surgical type, and operation time were also recorded. Result There were no statistically significant differences in baseline characteristics between the two groups (P 0.05). The D1 LUS in the CH group was significantly lower than that in the NS group (8.6 ± 2.7 vs. 10.4 ± 4.6, P 0.05). Similarly, the D3 LUS was also lower in the CH group compared to the NS group (6.7 ± 3.0 vs. 7.2 ± 3.0, P 0.05). Within the first three postoperative days, no statistically significant differences were observed between the two groups regarding PPC-related indicators, including pneumonia, fever, and cough (P 0.05). There were also no significant differences in Cdyn, OI, A-aDO2, or RI between the groups at T1, T2, or T3 (P 0.05). A subgroup analysis based on the presence of oral frailty was conducted among 44 elderly patients (24 in the NS group and 20 in the CH group). Among patients without oral frailty, the CH group showed a significantly lower D1 LUS than the NS group (P 0.05), while no significant differences were found in D0 LUS, D3 LUS, or PPCs (P 0.05). Among patients with oral frailty, the CH group demonstrated significantly lower D1 and D3 LUS compared to the NS group (P 0.05), whereas D0 LUS and PPCs showed no statistically significant differences between the groups (P 0.05). Conclusion Preoperative use of chlorhexidine mouthwash effectively improved postoperative lung ultrasound scores in elderly patients and may help reduce the risk of PPCs, particularly in those with oral frailty. Enhancing preoperative oral hygiene appears to be a beneficial perioperative strategy in the elderly population.