We aimed to assess the effect of chemoradiotherapy on the nutritional status of patients with nasopharyngeal cancer (NPC) and to detect the risk factors for poor nutrition status in NPC patients after radiotherapy. A total of 104 NPC patients participated in this clinical observational study. Psychological distress and nutritional indicators were measured prior to chemoradiotherapy. During the course of radiation therapy, side effect symptoms were assessed weekly. At the end of radiotherapy, nutritional indicators were measured again. Logistic regression was used to identify the risk factors for poor nutritional status after radiotherapy. The values of the 9 nutritional indicators were significantly lower after radiotherapy (P < 0.001) than the initial values before treatment. After radiotherapy, 20.19% of patients had more than 10% weight loss. At a significance level of α = 0.05, the risk factors for poor nutritional status were old age (P = 0.042), female gender (P < 0.001), late stage of the disease (P = 0.013), depression (P = 0.024), high side effect score (P = 0.007), and moderate nutritional status before radiotherapy (P = 0.015). Radiotherapy affects the nutritional status of NPC patients. To prevent malnutrition during radiotherapy, nutritional assessment and intervention should be an integral part of treatment.