Background Radiation-induced parotid gland damage is a common complication of radiotherapy (RT) in patients with nasopharyngeal carcinoma (NPC), which always causes xerostomia, dysphagia, dental caries, and even sleep disorders. Early evaluation of radiation-induced parotid damage is required to facilitate early interventions. Purpose To early-evaluate radiation-induced parotid damage using diffusion kurtosis imaging (DKI) in patients with NPC undergoing RT. Material and Methods Thirty-two patients with NPC underwent DKI for parotid glands pre-RT (two weeks before RT), mid-RT (five weeks after RT began), and post-RT (four weeks after RT). Parotid volume, apparent diffusion coefficient (ADC), corrected diffusion coefficient (D), excess diffusion kurtosis coefficient (K) values, mean radiation dose, and xerostomia degrees were recorded and analyzed. Results During RT, parotid ADC (change rates = 41.3 ± 25.2% at mid-RT, 70.8 ± 34.3% at post-RT) and D (change rates = 41.9 ± 25.2% at mid-RT, 63.2 ± 30.2% at post-RT) increased, while parotid volume (atrophy rates = 31.5 ± 7.9% at mid-RT, 30.6 ± 10.3% at post-RT) and K (change rates = –17.8 ± 11.0% at mid-RT, –29.8 ± 9.0% at post-RT) decreased significantly (all P < 0.001). At post-RT, the change rate of parotid D values was significantly positively correlated with the mean radiation dose ( r = 0.455, P < 0.001). Parotid ADC, D, and K values showed excellent intra- and inter-observer agreement (intraclass correlation coefficient = 0.946-0.985). Conclusion Radiation-induced parotid damage in patients with NPC undergoing RT could be effectively evaluated by DKI in the early stage.