Objective: This study aims to identify the factors influencing moderate to severe malnutrition in elderly lung cancer patients undergoing chemotherapy and to provide a basis for developing clinical prevention and intervention strategies. Methods: The clinical data of 150 elderly lung cancer patients who received chemotherapy were retrospectively analyzed. Demographic data, clinical characteristics, laboratory indicators, treatment plans, and other relevant information were collected. Independent risk factors for moderate to severe malnutrition during chemotherapy were identified. Results: During chemotherapy, 50 patients (33.33%) developed moderate to severe malnutrition. The BMI of patients with moderate to severe malnutrition was significantly lower than that of patients with no or mild malnutrition (21.20 ± 1.60 vs. 26.14 ± 2.31, P < 0.001), and the proportion of patients with stage IV tumors was significantly higher (60.00% vs. 27.00%, P < 0.001). Serum pre-albumin (pre-ALB) and hemoglobin (Hb) levels in patients with moderate to severe malnutrition were significantly lower than those in patients with no or mild malnutrition (152.67 ± 30.41 g/L vs. 252.47 ± 51.24 g/L and 102.44 ± 10.09 g/L vs. 154.21 ± 15.18 g/L, respectively, P < 0.001 for all). Conclusion: Low BMI, decreased serum pre-ALB levels, and decreased serum Hb levels before chemotherapy are independent risk factors for moderate to severe malnutrition in elderly lung cancer patients during chemotherapy. Close clinical attention should be given to these patients, with early intervention measures such as nutritional support to reduce the incidence of malnutrition and improve patients’ quality of life and prognosis.