Objective To evaluate the effect of inverse ratio ventilation on diaphragmatic function after thoracic lung surgery through M‑mode ultrasound. Methods A total of 86 patients, aged 18‒65 years, with body mass index (BMI) of 18.5 kg/m2 to 30.0 kg/m2, and American Society of Anesthesiologists (ASA) classification grades Ⅰ or Ⅱ, who were scheduled for unilateral single‑lung lobectomy, were selected. According to the random number table method, they were divided into two groups (n=43): a conventional ventilation group (group C) and an inverse ratio ventilation group (group F). After endotracheal intubation under general anesthesia, the tidal volume (VT) was set at 6‒8 ml/kg, respiratory rate (RR) at 12‒15 times/min, oxygen concentration at 50%, and oxygen flow at 1 L/min during double‑lung ventilation. For single‑lung ventilation, group F showed an inspiration‑to‑expiration ratio (I∶E) of 2∶1, with a VT of 6 ml/kg, and positive end‑expiratory pressure (PEEP) of 5 cmH2O (1 cmH2O=0.098 kPa), while group C had an I∶E ratio of 1∶2. Other respiratory parameters were the same between the two groups. Then, record patient demographics (age, gender ratio, BMI, ASA classification, hypertension, diabetes) and the heart rate, mean arterial pressure (MAP), arterial partial pressure of oxygen (PaO2), and arterial partial pressure of carbon dioxide (PaCO2) were recorded before anesthesia (T0), 5 min after single‑lung ventilation (T1), 1 h after single‑lung ventilation (T2), and immediately after surgery (T3). The airway peak pressure (Ppeak), plateau airway pressure (Pplat), mean airway pressure (Pmean), and PEEP were recorded at T1, T2, and T3. The M‑mode ultrasound was used to collect bilateral diaphragmatic excursion (DE) and diaphragmatic rapid shallow breathing index (D‑RSBI) at T0 and immediately after extubation (T4). Hospital stay duration, the incidence of diaphragmatic dysfunction and pulmonary complications within postoperative 5 d was recorded, and Spearman correlation analysis was conducted to evaluate the relationship between the incidence of diaphragmatic dysfunction and pulmonary complications within postoperative 5 d. Surgical duration, single‑lung ventilation time, and extubation time were recorded, as well as Observer's Assessment of Alertness/Sedation post‑extubation Results Compared with group C, group F showed increased DE and decreased D‑RSBI at T4 on both the operative and non‑operative sides (all P<0.05), reduction in DE differences and D‑RSBI differences (all P<0.05); increases in PaO2 at T1 and T2 (all P<0.05); reduction in Ppeak and Pplat at T1, T2, and T3 (all P<0.05), and increases PEEP and Pmean (all P<0.05). Compared with those at T0, both groups presented decreased DE and increased D‑RSBI at T4 on both the operative and non‑operative sides (all P<0.05). Compared with those at the operative side, both groups showed increased DE and decreased D‑RSBI at T4 on the non‑operative side (all P<0.05), and reduction in DE differences and D‑RSBI differences (all P<0.05). Spearman correlation analysis indicated that the incidence of diaphragmatic dysfunction was positively correlated with the incidence of pulmonary complications within postoperative 5 d (r=0.65, P<0.05). There were no statistical differences in other indicators (all P>0.05). Conclusions Inverse ratio ventilation can improve diaphragmatic function and reduce the incidence of diaphragmatic dysfunction. Diaphragmatic dysfunction is a key factor in postoperative pulmonary complications.