Objective To evaluate the relationship between preoperative red blood cell distribution width（ RDW） and postoperative cognitive dysfunction（ POCD） in coronary artery bypass grafting（ CABG）. Methods The clinical data of 362 consecutive patients with coronary heart disease undergoing CABG were retrospectively analyzed. The patients were divided into POCD group（ n=98） and non-POCD group（ n=264） according to the neuropsychological test 28 days after CABG. Demographic， past history， laboratory tests and perioperative data were compared between the two groups. Receiver operating characteristic（ ROC） curve was used to analyze the predictive value of preoperative RDW on POCD occurrence 28 days after CABG. Multivariate logistic regression analysis was used to analyze the relationship between RDW and POCD. Results The incidence of POCD 28 days after CABG was 27.1%（ 98 / 362）. RDW of patients with POCD was significantly higher than that of non-POCD patients［ （17.46±0.25） % vs（. 13.28±0.43） %，t=11.814，P＜0.001］. The area under ROC curve of RDW was 0.837， and the sensitivity and specificity were 82.7% and 64.8% respectively. High preoperative RDW（ OR=2.526， 95%CI：1.336-4.776，P ＜ 0.001）， presence of aortic atherosclerotic plaque（ OR=1.461，95%CI：1.064-2.007，P=0.037）， history of cerebrovascular disease（ OR=2.874， 95%CI： 1.683-4.908，P＜0.001）， high fasting blood glucose level（ OR=1.196，95%CI： 1.078-1.327，P=0.041）， high sensitivity C-reactive protein（ OR=1.267，95%CI：1.029-1.560，P=0.024） and long operation time（ OR=2.123，95%CI： 1.671-2.696，P=0.001） were independent risk factors for POCD. Conclusions The increase of RDW before CABG is a risk factor for POCD 28 days after CABG， and can be used as a predictor.