Objective To explore the effect of bright light therapy combining conservative oxygen therapy on prevention of delirium in critically ill patients on mechanical ventilation. Methods Three hundred and sixty mechanically ventilated patients in the ICU were randomized into three groups, with 120 cases in each group: bright light therapy at an intensity of 100 Lux (placebo group), bright light therapy at an intensity of 10 000 Lux (control group), and bright light therapy combining conservative oxygen therapy (combination group). The incidence of delirium, duration of delirium, drug intervention on delirium, delirium-related adverse outcomes (unplanned extubation, ICU length of stay and ICU mortality) and sleep disorders were compared among the three groups. Results The incidence rates of delirium were 26.66%, 28.57% and 16.67% in the placebo group, control group and combination group respectively. There were significant differences in duration of delirium and sleep disorder scores among the three groups (P<0.01 for both), with the combination group superior to the other two groups. Conclusion Bright light therapy in combination with conservative oxygen therapy can decrease delirium, shorten duration of delirium, and enhance sleep quality in mechanically ventilated patients in the ICU.