Objective This study aimed to analyze the correlation between the third lumbar skeletal muscle index L3 -SMI and nutritional status and to clarify whether L3 -SMI could be used as an effective predictor of malnutrition for advanced non-small cell lung cancer NSCLC patients. Method A total of 100 clinical stage Ⅲ - Ⅳ NSCLC patients who received radiotherapy and / or chemotherapy in Funing People's Hospital from Jan 2016 to Dec 2020 were enrolled in this analysis and all patients were divided into severe malnutrition and non-severe malnutrition groups based on the patient-generated subjective global assessment PG-SGA . The correlations between different assessment parameters and severe malnutrition risk were determined using the univariate and multivariate Logistic regression analysis and the receiver operation characteristics ROC curves were plotted to compare these predictive values for severe malnutrition risk in advanced NSCLC patients. Result Using the PG-SGA scale we found that frequency of severe malnutrition in advanced NSCLC patients could be as high as 48. 0% . Compared with the non -severe malnutrition group patients with severe malnutrition were older 65. 4± 10. 0 vs 61. 2 ± 6. 9 years old t = 2. 482 P = 0. 015 and had a higher score of Controlling nutritional status 3 1 5 vs 2 1 4 t = 2. 238 P = 0. 027 lower body mass index 21. 1±3. 2 kg / m 2 vs 25. 2±3. 8 kg / m 2 t = -5. 840 P<0. 001 serum albumin level 33. 8±4. 7 vs 36. 4±3. 9 g / L t = -2. 970 P= 0. 004 the prognostic nutritional index 41. 5±5. 3 vs 44. 7±5. 0 t = -3. 122 P= 0. 002 and L3 -SMI 36. 0±8. 9 vs 53. 1±12. 3 t = -8. 103 P<0. 001 . The multivariate Logistic regression analysis demonstrated that low serum albumin level OR = 0. 791 95%CI = 0. 678- 0. 922 P = 0. 003 and L3 -SMI OR= 0. 826 95%CI = 0. 765-0. 892 P<0. 001 were independently correlated with severe malnutrition risk in advanced NSCLC patients. In addition ROC curves showed that L3 -SMI had an excellent predictive performance with an AUC value of 0. 888 95%CI = 0. 822 - 0. 955 which was higher than other hematological parameters. Conclusion L3 -SMI was significantly correlated with PG-SGA scale and it was an useful tool of nutrition risk screening for advanced NSCLC patients.