Ｏｂｊｅｃｔｉｖｅ　To observe the effect of permissive hypercapnia(PH) on postoperative cognitive function in patients with carotid endarterectomy(CEA).　Methods　Forty patients who were selected for unilateral CEA were divided into two groups, aged 40-70 y, sex unlimited, American Society of Anesthesiologists(ASA) grade Ⅱ-Ⅲ. The patients were divided into two groups by random numerical table method：conventional ventilation group (group R) and PH group (group H). The parameters of mechanical ventilation in group R with hypercapnia ventilation were as follows: tidal volume (VT) was 8-10 ml/kg, respiratory rate (RR) was 12-16 bpm, arterial partial pressure of carbon dioxide (PaCO2) was 35-45 mmHg (1 mmHg=0.133 kPa), the parameters of mechanical ventilation in group H were set as follows: VT was 6-8 ml/kg, RR was 12-14 bpm, PaCO2 was 46-55 mmHg. The basic indexes such as operation time, carotid artery occlusion time, extubation time and post-anesthesia care unit (PACU) stay time were recorded. Cerebral oxygen saturation (rSO2) was monitored by near infrared spectroscopy (NIR). The systolic blood pressure (SBP), diastolic blood pressure (DBP), mean arterial pressure (MAP), heart rate, saturation of pulse oximetry (SpO2), end-tidal carbon dioxide partial pressure (PETCO2) and rSO2 were recorded before operation (T0), immediately after anesthesia induction (T1), 10 min after anesthesia induction(T2), immediately before carotid artery occlusion (T3), immediately after carotid artery occlusion (T4), and at the end of operation (T5). The cognitive function of the patients was evaluated by Mini-Mental State Examination (MMSE) scores at T0, 24 h, 48 h, 3 d and 7 d after operation.　Results　There were no significant difference in operation time, carotid artery occlusion time, extubation time and PACU residence time between the two groups(P＞0.05). Compared with group R, PETCO2, rSO2 and MMSE score increased in group H at the time point of T2-T5, the MMSE score increased at 24 h, 48 h after operation, which alleviated the occurrence of postoperative cognitive dysfunction (POCD) in cognitive impairment after operation (P<0.05). Compared with T0, the rSO2 at T2-T5 time point in two groups was lower (P<0.05), MMSE scores decreased significantly at 24 h, 48 h, 3 d and 7 d after operation(P<0.05).　Conclusions 　PH ventilation can improve cerebral oxygen metabolism and reduce the occurrence of POCD after operation.