Objective Critical weight loss not only reduces quality of life, but also affects prognosis. This study validated its effectiveness in reducing the incidence of critical weight loss during proton and heavy ion therapy by implementing a guideline⁃based nutrition education and counselling program. Methods Patients during proton heavy ion therapy were recruited, and a historical control study design was adopted. The patients treated from Jan to Dec 2016 were used as the control group, and the patients from Jan to Aug 2018 as experimental group. The control group still received the original routine care, and the experimental group was given a nutrition education and counselling program based on the guidelines. Results A total of 713 cancer patients were included in the study, including 374 patients in the control group and 339 patients in the experimental group. The average age of the two groups was 54 and 53 years, and 45% and 49% were patients with head and neck cancer. The average weight loss during radiotherapy was 0.51 kg (0.75%) in the control group, and the rate of critical weight loss was 11.2% (42 cases); the experimental group lost 0.66 kg (0.90%), and the rate of critical weight loss was 9.4% (32 cases). After controlling the confounding factors of total radiation dose, radiation type, tumor location, concurrent chemotherapy, and gender, the risk of significant weight loss in the experimental group decreased by 34% (OR=0.66, 95%CI=0.48-0.91). Conclusion Guideline⁃based nutrition education and counselling programs can help improve the nutritional status of patients during proton and heavy ion therapy and effectively reduce the risk of significant weight loss