A prospective pilot study investigated the relationship between FiO2 and PaO2 in 50 patients during general anesthesia with dual lung ventilation. FiO2 started at 0.3 after intubation and increased in steps of 0.1 up to 0.8, with each step lasting at least 30 min before blood gas sampling. The analysis showed a significant linear correlation between FiO2 and PaO2 with a coefficient of r=0.967 (p<0.001). At FiO2 of 0.4, 78% of patients maintained PaO2 between 100 and 200 mmHg, while at 0.5, more than 68% of patients exceeded 200 mmHg. There was also a weak correlation between FiO2 and the PaO2/FiO2 ratio (r=0.290; p<0.001). The study suggests that an FiO2 of 0.4 is sufficient for most patients at a PaO2 of 100–200 mmHg unless there are specific clinical requirements, and usually does not need to exceed 0.5. The study is registered under ChiCTR2500095383.