Objective To explore the suitable fresh oxygen flow rate for the rise of lung oxygen concentration in patients under general anaesthesia with tracheal intubation. Methods Sixty ASA I or II patients, aged 18 ~ 60yr, BMI 18.5 ~ 25, scheduled for elective surgery under total intravenous anesthesia and volume control mechanical ventilation，were enrolled in this study. Firstly, the fraction concentration of inspired oxygen（FiO2）was adjusted to 30% and the minite flesh gas flow rate 0.1L/kg. Preoxygenation was undertaken when the fractional concentration of expired oxygen（FeO2） reached 30%: The minite flesh oxygen flow rate（FO2） was adjusted to 0.02 L/kg、0.04 L/kg、0.06 L/kg、0.08 L/kg、0.1 L/kg、0.12 L/kg、0.14 L/kg and 0.16 L/kg in turn and FiO2 was adjusted to 100%. The times of rised FeO2 from 30% to 50%（t30% ~ 50%） was compared under different FO2,.the required flesh oxygen volume under different FO2 was calculated, and Pearson correlation between FO2 and VO2 was analyzed. Results ①t30% ~ 50% was respectively 181 s, 97 s, 68 s, 58 s, 53 s, 49 s, 47 s and 44 when the oxygen concentration in the lung was respectively increased in a FO2 of 0.02 L/kg, 0.04 L/kg, 0.06 L/kg, 0.08 L/kg, 0.1 L/kg, 0.12 L/kg, 0.14 L/kg and 0.16 L/kg. ②There was significant decreases in different t30% ~ 50% when FO2 was 0.02 L/kg、0.04 L/kg and 0.06 L/kg（P <0.01）; There was significant decrease between t30% ~ 50% when FO2 was 0.02 ~ 0.06 L/kg and 0.08 ~ 0.16 L/kg（P <0.01）, significant difference also existed between t30% ~ 50% when FO2 was 0.08 L/kg and 0.16 L/kg（P <0.01）. ③The change in VO2 was positively correlated with FO2(r = 0.864，P <0.01). Conclusion A FO2 of 0.08 ~ 0.1 L/kg can rapidly increase the FeO2 from 30% to 50% for patients under general anaesthesia with tracheal intubation, while not too much flesh oxygen is required.