Objective To investigate the dynamic changes of dead space fraction (VD/VT) and its effect on the prognosis in the patients with acute respiratory distress syndrome (ARDS).Methods Thirtytwo patients with ARDS in EICU of the First Affiliated Hospital of Kunming Medical University from January 2015 to August 2016 were selected and divided into the survival group and the death group according to the 28 d mortality rate.VD/VT on 1-6 d in the two groups was calculated according to the formula.The parameters of basic condition,PaO2/FiO2,APACHE Ⅱ score,LIPS score,PEEP of the two groups were compared.Results LIPS and APACHE Ⅱ scores had statistical difference between the death group and survival group (P＜0.05);VD/VT on 1-3 d had no statistical difference between the two groups (P＞0.05),and VD/VT on 4-6 d in the death group was significantly higher than that in the survival group (P＜0.01).△PCO2,pH and PaO2/FiO2 had statistical difference between the death group and the survival group (P＜0.05).The binary classification Logistic regression analysis found that APACHE Ⅱ,LIPS,△PCO2,VD/VT were the risk factors.VD/VT on the 4th day was selected,its' cut off value was 0.62,and the sensitivity of the prognostic evaluation was 90.0 ％,the specificity was 95.5 ％.Conclusion The dead space fraction is an independent risk factor for the prognosis of ARDS patients,and VD/VT on 4th day could effectively identify the high-risk patients.