Objective To evaluate the characteristics of quantitative monitoring of brain function during the perioperative period in elderly patients and its relationship with postoperative cognitive dysfunction (POCD).Methods Seventy ASA physical status Ⅰ or Ⅱ patients,aged ≥ 60 yr,scheduled for elective lumbar spine decompression and fusion surgery under general anesthesia,having an expected postoperative length of hospital stay ≥ 7 days,were enrolled in the study.The cognitive function was assessed by using Mini-Mental State Examination before operation and the results were normal.Fifty healthy elderly volunteers were chosen and served as control group.Cognitive function was assessed at l day before operation (D0) and 3 (D3) and 7 days after operation (D7).Z score was used to identify POCD.All the patients were then divided into POCD group or control group (group C) according to the results of diagnosis.Quantitative monitoring of brain function was carried out using a traction system,and the wavelet index (WLI),i_22 and i_20 were recorded.Results A total of 67 patients completed the study and were enrolled in the analysis,there were 9 cases in group C,and 58 cases in group POCD.The WLI was significantly decreased at D7,and no significant change was found in WLI at D3 as compared with the value at D0.The WLI was significantly lower at D7 than at D3.There was no significant difference in i_22 and i_20 between the three time points.Compared with group C,i_22 was significantly decreased at D0,and no significant change was found in i_22 at the other time points and in WLI at each time point in POCD group.Conclusion During quantitative monitoring of brain function during the perioperative period in the elderly patients,WLI is significantly decreased on 7th day postoperatively,and no significant change is found in i_20 and i_22,however,the pre-operative low i_22 value can predict the development of POCD.