Objective Bioelectrical impedance analysis BIA was used to observe the fluid distribution of patients with head and neck cancer who were undergoing radiotherapy including intracellular water extracellular water total body water ECW/ ICW % ECW/ TBW % changes. At the same time by observing the occurrence time and degree of radiation oral mucositis ROM during radiotherapy the correlation between the fluid distribution and the occurrence and degree of ROM was analyzed. Method A total of 60 patients with head and neck caner who were undergoing radiotherapy in the Radiotherapy Department of Jiangsu Province Hospital of Chinese Medicine from September 2022 to March 2024 were prospectively included. fluid distribution indicators such as ICW ECW TBW ECW/ ICW % and ECW/ TBW % were collected and recorded before at the 2nd 4th and 6th week and finished day of radiotherapy respectively. To observe the occurrence time and severity of oral mucositis during radiotherapy RTOG radiation damage grading standard and explore the correlation between them. Result The values of ICW ECW and TBW varied with the degree of ROM at the 4th week of radiotherapy. ICW values in patients with OM grade 3 or higher were significantly lower than those with OM grade 2 or lower P < 0. 05 while ECW ECW/ ICW % and ECW/ TBW % values were increased significantly P < 0. 05 . No correlations were found between fluid distribution and ROM occurrence in other time. Conclusion The fluid distribution of patients with head and neck cancer during radiotherapy was significantly related to the occurrence and degree of radiation oral mucositis. The characteristics of human fluid distribution based on BIA measurement may be used as a potential predictor of radiation-induced oral mucositis which has great clinical significance for screening patients with high risk of ROM and prior intervention before symptoms occur.