【Abstract】 Objective To explore the effects of anesthetic depth regulated by AAI and BIS on cognitive function in gerontal patients suffering total knee replacement surgery under general anesthesia. Methods Sixty old patients undergoing total knee replacement surgery , aged 65-75 yr , weighing 45-85 kg, ASA physical status Ⅰor Ⅱ; The random digits table method was used to divide patients into AAI group, BIS group and contract group(CON group), and 20 cases were dispatched randomly in each group. At 1 day before and 1 day, 3 day, 5 day, 7 day after operation, cognitive function were assessed by Mini-Mental State Examination and the scores were recorded . The recovery time of patients after operation, extubation time, the observer’s assessment of alertness/sedation(OAA/S) scale and visual analogue scale (VAS) were recorded. the total drug dosage during general anesthesia were also recorded. Results Compared with group CON[Propofol(810±17), remifentanil(1.26±0.07)mg], the propofol dosage [ Group AAI(450±19), Group BIS(500±18)mg]and the remifentanil dosage[Group AAI(0.78±0.07), Group BIS(0.80±0.08)mg ] were less, and group AAI was more less than Group BIS(P<0.05). The patient's awakening time [Group AAI (5.4±1.5), Group BIS(10.2±1.3)min] and the extubation time [Group AAI (7.3±1.6), Group BIS(14.5±1.5)min ] were shorter than that of Group CON [Awakening time (15.3±1.8), Extubation time(18.2±1.7)min ] (P<0.05), and the alertness sedation score [Group AAI(4.45±0.35), Group BIS(3.74±0.43)] were higher than that of Group CON(2.85±0.24)(P<0.05). There were no significant differences among three groups in the MMSE scores of each test used for assessment of cognitive function before operation. And there were no significant differences about the scores of MMSE of every group from the first day to the seventh day(P>0.05). Conclusions Application of anesthetic depth monitoring can reduce the dosage of anesthetic, shorten the awakening time and extubation time, but have no significant effect on the postoperative cognitive function scale (MMSE).