The purpose of this study was to use a 3-dimensional arterial spin labeling (3D ASL) magnetic resonance (MR) method to measure cerebral blood flow (CBF) before and after radiotherapy, and correlate changes with time after receiving radiotherapy and cognitive function. Patients with nasopharyngeal carcinoma receiving radiotherapy at our institution were recruited for the study. Participants were divided into three groups: Pre-radiotherapy control (PC) group, acute reaction period (ARP) group, and delayed reaction period (DRP)group. Thirty-four patients were included in the study. Compared with the PC group, the ARP group exhibited significantly decreased perfusion in the left anterior cingulate cortex (ACC) and right putamen, and increased perfusion in the right cerebellum (Crus 1), right inferior occipital gyrus, left lingual gyrus, left precuneus, and left calcarine gyrus. in the DRP group, increased perfusion was noted in the right cerebellum (Crus 1) and decreased perfusion in the left superior frontal gyrus. CBF differences were observed in several brain areas in the DRP group as compared to the ARP group (P < 0.001). Total Montreal Cognitive Assessment score, and subdomain language and delayed memory recall scores were significantly lower in the ARP and DRP groups than in the PC group (P < 0.05). Data suggest that ASL allows for non-invasive detection of radiation-induced whole-brain CBF changes, which is transient, dynamic and complicated and may be a factor contributing to cognitive impairment induced by radiotherapy for nasopharyngeal carcinoma.