Objective To investigate whether patients with mechanical ventilation should be routinely given preoxygenation before closed sputum suction. Methods Twenty-seven patients were selected. In the before-after self-controlled design, each patient was given preoxygenation(preoxygenation group) and without preoxygenation(non-preoxygenation group) before sputum suction. SpO2, heart rate and mean arterial pressure were recorded before sputum suction, immediately after sputum suction and 1 min after sputum suction. Results SpO2, heart rate and mean arterial pressure over varied time points had no significant group effect (P>0.05), but had significant time effect (P<0.05,P<0.01); group by time effect was significant for SpO2, but not significant for heart rate or mean arterial pressure (P>0.05 for both). Conclusion For patients with normal SpO2, routine preoxygenation is not recommended before closed sputum suction, and the physiological changes of patients should be closely observed during sputum suction. At the time of sputum suction, it is still recommended to give preoxygenation before closed sputum suction for patients who may have significantly decreased blood oxygen saturation from such conditions as cardiac insufficiency, airway malocclusion, and bronchial asthma, etc., so as to improve oxygen reserve and ensure patient safety.