Objective To investigate the clinical significance and levels of plasma renin activity (PRA), Angiotensin (Ang) Ⅱ in ALI / ARDS patients. Methods The levels of PRA and AngⅡ, APACHE Ⅱ score, lung injury score(LIS), and mortality were analyzed in ALI/ARDS patients and non-ALI/ARDS group. After the patients were grouped accord-ing to prognosis, correlation analysis and ROC curve to evaluate diagnostic efficacy of PRA and AngⅡ were performed. Results Compared with non- ALI/ARDS group, the levels of PRA and AngⅡ were significantly increased in ALI/ARDS (P<0.05). In the death group, the levels of PRA and AngⅡwere higher than that of the survival group (P<0.05). It was significant positive correlation between APACHEⅡ score,LIS,lactic acid (LAC) and PRA, so as AngⅡ. In pre-dicting ALI/ARDS, PRA was significant (AUC=0.705, P=0.001), so as AngⅡ(AUC=0.672, P =0.007). In predict-ing ALI/ARDS, there was no significant difference between PRA and LIS (P=0.923), and also between AngⅡ and LIS (P = 0.713). Conclusion In ALI/ARDS patients, the levels of PRA and AngⅡwere significantly increased, and both could be used to predict ALI/ARDS and determine its prognosis, but AngⅡhad higher specificity in predicting ALI/ARDS.