Abstract: Objective:To investigate the relationship between APACHE Ⅱ score and weaning outcome of mechanical ventilation in patients with severe pneumonia.Methods:The APACHE Ⅱ scores of 45 severe pneumonia patients treated with mechanical ventilation were obtained before spontaneous breathing test(SBT) at the first day of admission and according with weaning standard.According to the weaning outcome,the patients were divided into the observation group(35 cases) and control group(10 cases).The observation group did not need ventilator-assisted ventilation after 48 hours of pulling tracheal intubation,and the control group need reintubation to recover ventilator-assisted ventilation within 48 hours of extubation and after the SBT failure.The APACHE Ⅱ score at the first day of admission and before weaning,and mechanical ventilation time before SBT between two groups were compared.The receiver operating characteristic curve (ROC curve) was drew,and the area under curve(AUC) was calculated.The APACHE Ⅱ score was used to predict the weaning outcome.Results:The differences of the APACHE Ⅱ score at the first day of admission,mechanical ventilation time before SBT and APACHE Ⅱ score before weaning between two groups were not statistically significant(P>0.05).The AUC of the ROC curve of APACHE Ⅱ score was 0.871,the best cut-off value was the APACHE Ⅱ for 10.1,and the sensitivity and specificity was 90.6% and 53.8%,respectively.Conclusions:APACHE Ⅱ score has a certain value in predicting the outcome of severe pneumonia,and can be used as a reference index in weaning of mechanical ventilation.