Malnutrition was prevalent in cancer patients with incidence ranging from 40% to 80%. About 97% of the patients needed nutritional therapy. Malnutrition in oncology patients might be caused by multiple factors, including the psychological changes of tumor, response to the tumor and cancer treatment. About 70% of tumor patients prescribed radiotherapy in the course of therapy. However several acute and chronic responses accompany during and after the radiotherapy, which might easily lead to malnutrition in patients and cause therapy interruption and delay. This could have an impact on the quality of the patients’ lives and the therapeutic effect, having adverse effects on prognosis. Nutritional therapy could help radiotherapy patients pass through adverse reaction stage, reduce the toxic reaction caused by radiotherapy, improve phycical condition and reduce the occurrence of adverse events. Therefore, we should reinforce the nutritional status assessment during and after the radiotherapy scientifically and comprehensively and give individual nutritional therapy as early as possible. These might improve the nutritional status and quality of life of patients, which could not only help anticancer therapy, but also reduce the risks of tumor progression and improve the outcomes. This article summarized the malnutrition problems resulting from radiotherapy to the body and the influence of nutritional therapy on radiotherapy effect and physical condition of patients. The article suggested nutritional therapy plays an important role in improving quality of life and prognosis of patients by reviewing the application of nutritional therapy in patients with radiotherapy.