Objective To investigate the trend of critical weight loss during radiotherapy in head and neck cancer patients and to analyze its influencing factors. Method A historical cohort study design was used. Head and neck cancer patients treated with radiotherapy in Xiangya Hospital Central South University were enrolled for the research from September 2021 to October 2023. A self-developed data collection tool was used to collect information about the study subjects before radiotherapy T0 at week 1 T1 week 2 T2 week 3 T3 week 4 T4 week 5 T5 and week 6 T6 after the start of radiotherapy. Factors influencing the trajectory of critical weight loss were analyzed using generalized estimated equation. Result A total of 575 head and neck cancer patients were included in this study and the incidence of critical weight loss was 1. 22% 6. 43% 14. 61% 24. 87% 37. 39% and 52. 52% at T1 T2 T3 T4 T5 and T6 respectively. The generalized estimated equation analysis revealed that clinical stage with stageⅠ as reference ORⅡ = 3. 685、 ORⅢ = 3. 520、 ORⅣ = 3. 792 baseline BMI with BMI < 18. 5 kg / m 2 as reference OR24. 0~ 28. 0 kg / m 2 = 2. 832、OR≥28. 0 kg / m 2 = 3. 481 and albumin level OR = 0. 943 were significant predictors P< 0. 05 for critical weight loss trajectories during radiotherapy in head and neck cancer patients. Conclusion The incidence of critical weight loss in head and neck cancer patients is on the rise with the prolongation of radiotherapy cycles. Healthcare professionals should dynamically monitor weight changes in head and neck cancer patients during radiotherapy with prioritized implementation of multi - level nutritional interventions for patients with advanced-stage disease high BMI or low albumin levels to reduce the incidence of malnutrition.