ObjectiveTo investigate acute physiologic and chronic health evaluation Ⅱ(APACHE Ⅱ) score system for severity evaluation and prognosis prediction of patients undergoing cardiac surgery. MethodsA total of 3 566 patients who were admitted in ICU after cardiac surgery in the Department of Cardiovascular Surgery of General Hospital of Shenyang Military between December 1, 2011 and August 31, 2013 were enrolled in this study. There were 1 873 males and 1 693 females with their average age of 45.8±23.7 years (range, 10 days to 82 years). All the patients were evaluated with APACHE Ⅱ and expected mortality was calculated. Receiver operating characteristic(ROC) curve was drawn to compare expected and actual mortality and evaluate predictive value of APACHE Ⅱ. ResultsA total of 3 373 patients survived the operation, and 193 patients died postoperatively with the mortality of 5.41%. The area under the ROC curve was 0.917 (P=0.000) with 95% confidence interval of[0.885,0.949]. The cut-off point of APACHE Ⅱ was 15.50, with a sensitivity of 80.3%,a specificity of 95.6%,an accuracy rate of 79.5%,a positive predictive value of 86.9%,and a negative predictive value of 93.1%. Expected and actual mortality increased with increasing APACHE Ⅱ,which were both positively correlated. ConclusionAPACHE Ⅱ score system can be used to evaluate severity and predict prognosis of patients undergoing cardiac surgery, which provides reference for rational utilization of ICU resource.