Objectives: Intensity-modulated radiation therapy is the cornerstone of curative treatment for nasopharyngeal carcinoma; however, radiation-induced brain injury has an insidious onset, with no obvious morphological abnormalities in brain tissue in the early stages. It is difficult to detect through routine imaging examinations, making it easy to miss the optimal window for intervention. To investigate the dynamic progression of early radiation-induced brain injury following radiation therapy for nasopharyngeal carcinoma, identify the optimal evaluation window, and determine the correlation between tumor staging and brain injury, this study conducted a dynamic follow-up study using multimodal MRI technology. Methods: Thirty patients with newly diagnosed nasopharyngeal carcinoma were enrolled and divided into three groups based on TNM staging: T1/T2, T3, and T4, with 10 patients in each group. All patients underwent radical intensity-modulated radiation therapy. Multimodal MRI scans were performed 1 day before radiotherapy and 1, 3, and 7 weeks after radiotherapy. Changes in the microstructure of the temporal lobe and hippocampal white matter, as well as brain functional network parameters, were quantitatively compared across groups. Results: One week after radiotherapy, only physiological fluctuations in brain tissue parameters were observed, with no pathological damage; three weeks after radiotherapy, subclinical white matter structural disorganization and abnormalities in brain functional networks could be consistently detected, representing the optimal time window for assessing early damage; seven weeks after radiotherapy, brain damage showed cumulative worsening in proportion to radiation dose. The overall detection rate of early brain injury in this group was 46.67%. The incidence and severity of brain injury in stage T4 patients were significantly higher than those in patients with early- to mid-stage disease (P < 0.05). The FA, ReHo, and ALFF parameters measured in the temporal lobe 3 weeks after radiotherapy demonstrated good diagnostic performance for early brain injury. Conclusion: Multimodal MRI can accurately detect occult early-stage radiation-induced brain injury following radiotherapy for nasopharyngeal carcinoma. Abnormal changes in brain function occur earlier than morphological damage to brain tissue, and TNM staging is positively correlated with the severity of brain injury. This technique provides reliable imaging evidence to support the early implementation of neuroprotective interventions and the optimization of individualized radiotherapy regimens.