Background The effect of dexmedetomidine on cognitive function after various surgeries were reported, however no consensus is made on pulmonary surgery. In this study we aimed at investigating the effect of dexmedetomidine anesthesia on postoperative cognitive function (POCD) in pulmonary surgery. Methods A prospective randomized placebo-controlled study was conducted with blinded to patients. The study was performed on 60 patients (29 in the dexmedetomidine group; 31 in the placebo group). Dexmedetomidine-group patients received dexmedetomidine (1 µg/kg, i.v.) and Placebo-group patients received an equal volume of physiologic (0.9%) saline for 20 min before anesthesia induction. Cognitive function was evaluated using Montreal Cognitive Assessment (MoCA) 1 day before surgery, as well as on postoperative day (POD)1, POD3 and POD7. The regional cerebral oxygen saturation (rSO2) was monitored continuously by near-infrared spectroscopy before anesthesia. Results The MoCA score between the two groups was significantly different on POD1 (Dex 26.4 ± 0.73 vs Placebo 25.5 ± 0.96, p < 0.001) and POD3 (Dex 27.1 ± 0.79 vs Placebo 26.6 ± 0.80, p = 0.032). In detail, attention and orientation scores were increased in the dexmedetomidine group on POD1 and POD3. The rSO2 between the dexmedetomidine group or placebo group was not significantly different (Dex 64.9 ± 2.73 vs. Placebo 64.3 ± 3.29, p = 0.483), and no significant difference was found before drug administration and after regaining consciousness (Dex 63.5 ± 2.52 vs. Placebo 64.2 ± 3.22; p = 0.390). Conclusion We showed, for the first time, that dexmedetomidine (1.0 µg/kg) could reduce the risk of POCD and might not decrease rSO2. Hence, dexmedetomidine could be employed in pulmonary surgical procedures, especially for older patients facing high risk of delirium.