Objective This study aimed to explore the optimal threshold and predictive value of skeletal muscle index at the third lumbar vertebra level L3-SMI for the prognosis of esophageal cancer patients. Method 150 esophageal cancer patients who were treated with surgical resection or curative chemoradiotherapy in Haian Hospital Affiliated to Nantong University from January 2016 to October 2020 were analyzed retrospectively. The L3-SMI was measured by analyzing abdominal computed tomography CT images. The optimal cutoff values of L3 - SMI for survival prediction of male and female patients were identified by the receiver operation characteristics ROC curves respectively. All patients were divided into high and low L3-SMI groups and Kaplan-Meier curve was used to estimate the 3-year overall survival OS rate of the two patient groups. In addition the independent prognostic factors of L3-SMI for esophageal cancer patients were further determined by the univariate and multivariate Cox regression analysis. Result In this study cohort the average value of L3-SMI for male and female patients were 43. 8±6. 3 cm 2 / m 2 and 38. 7±6. 7 cm 2 / m 2 respectively. The best cutoff values of L3-SMI for survival prediction of esophageal cancer patients were <42. 8 cm 2 / m 2 for males and <39. 2 cm 2 / m 2 for females respectively. The AUC value of the former was 0. 628 95% CI = 0. 515- 0. 742 and the latter had an AUC value of 0. 654 95%CI = 0. 513-0. 796 . All patients were divided into high L3-SMI n = 79 and low L3-SMI groups n = 71 based on the identified cutoff values. The results of survival analysis indicated that the 3-year OS rate of low L3-SMI and high L3-SMI group were 33. 5% and 54. 6% with a significant statistical difference χ 2 = 5. 358 P= 0. 021 . The subgroup analysis showed that low L3-SMI was significantly associated with worse OS in male patients 3-year OS rate 39. 8% vs 65. 4% χ 2 = 4. 752 P= 0. 029 . For female patients with esophageal cancer low L3-SMI group had a lower 3-year OS rate than high L3-SMI group 39. 8% vs 65. 4% but the survival difference was not statistically significant χ 2 = 1. 835 P= 0. 176 . The results of the univariate and multivariate Cox regres-sion analysis demonstrated that low L3-SMI HR= 1. 669 95%CI = 1. 062-2. 625 P= 0. 027 prognostic nutritional index PNI < 50. 2 HR=1. 700 95%CI = 1. 033-2. 796 P = 0. 037 and Ⅲ stage of the TNM stage HR = 1. 503 95%CI = 1. 077-2. 479 P = 0. 024 were independent prognostic factors for esophageal cancer patients. Conclusion L3-SMI is a valuable imaging parameter of prognostic assessment for esophageal cancer patients.