Objective This study aimed to explore the role of fat-free mass index FFMI measured by bioelectrical impedance analysis BIA for nutritional and prognostic assessment of non - metastatic nasopharyngeal carcinoma patients. Method This retrospective study collected clinicopathological data of 162 nasopharyngeal carcinoma patients who received systematic treatment in Hai'an People's Hospital From January 2019 to June 2022 and FFMI was measured and calculated by BIA. All patients were divided into low FFMI n = 55 and normal FFMI group n = 107 according to its threshold recommended by Asian Working Group of Sarcopenia male FFMI <17 kg / m 2 female FFMI <15 kg / m 2 . The baseline characteristics and progression-free survival PFS of the two groups were compared. Kaplan-Meier curves and the Cox proportional regression model were used to evaluate the relationship between FFMI and the prognosis of nasopharyngeal carcinoma patients. Result The mean FFMI of the entire cohort was 17. 8 ± 2. 0 kg / m 2 . Of them 34. 0% 55 / 162 were diagnosed as low FFMI. The patients with low FFMI had a lower BMI t = 5. 168 P<0. 001 and higher risk of malnutrition NRS 2002≥ 3 points χ 2 = 5. 817 P = 0. 015 than those with normal FFMI. With regard to nutrition-related parameters the level of serum albumin t = 2. 032 P= 0. 044 lymphocyte count t = 2. 674 P= 0. 008 mid-arm circumference t = 3. 850 P<0. 001 and handgrip strength t = 2. 229 P= 0. 027 in patients with low FFMI were significantly lower than in those with normal FFMI. The Kaplan - Meier curve showed that low FFMI was correlated with poor prognosis of nasopharyngeal carcinoma patients. The 3-year PFS rates of patients with low and normal FFMI were 45. 6% and 68. 4% respectively χ 2 = 7. 957 P= 0. 005 . The univariate and multivariate Cox regression analysis revealed that TNM stage HR= 3. 393 95%CI = 1. 911-6. 023 P<0. 001 and FFMI HR = 2. 182 95% CI = 1. 242 - 3. 831 P = 0. 007 were independent predictor of PFS for nasopharyngeal carcinoma patients. Conclusion The FFMI measured by body composition analysis had important clinical significance for nasopharyngeal carcinoma patients and it might be a useful parameter for nutritional and prognostic assessment.