To examine the adrenal function of critically ill patients received mechanical ventilation, and explore the relationship between the relative adrenal insufficiency(RAI) and weaning outcome, evaluate the impaction of RAI impact on prognosis. Critically ill patients who were mechanical ventilated over 24 hours were enrolled in this study. Each patient was given a bolus dose of 250 ��g corticotrophin, plasma total cortisol level was detected at T0 and T60 (before and 60 minutes) after the shot and RAI defined as the difference between T0 and T60��9 ��g/dl. Chi-square test was used to analysis the correlation of the RAI and the weaning outcome, and Kaplan-Meiler was used evaluate the 30 days survival. Results A total of 60 patients with mechanical ventilation were recruited, including 38 invasive ventilation and 22 non-invasive. The successful weaning group consisted 24 patients and the failure weaning group was 36. The Chi-square test showed whether the incidence of RAI was significant correlation with weaning outcome (P=0.000 11), and more obvious in invasive group. Observation of the rate of 30 days survival rate were observed by the Kaplan-Meiler, it showed mortality of RAI incident in ventilated patients was significantly higher than non-RAI group(P = 0.000 3). The occurrence of RAI significantly affects the rate of weaning in invasive ventilated patients; and is significantly associated with the mortality of ventilation.