Objective To analyze the factors influencing the deterioration of nutritional status after radiotherapy/radiochemotherapy for esophageal cancer, so as to provide reference for nutritional management during antitumor therapy. Methods A total of 106 patients with esophageal cancer who received radiotherapy or radiochemotherapy at Anqing First People’s Hospital of Anhui Medical University from Dec. 2017 to Dec. 2023 were enrolled. Patients’ gender, age, surgical history, timing of radiotherapy intervention, synchronous chemoradiotherapy, radiotherapy dose, clinical stage, initial nutritional status, and performance status score were collected. The patient generated subjective global assessment scale (PG-SGA) scores were monitored before and after antitumor treatment. According to the nutritional status at the beginning of enrollment and at the end of radiotherapy, the patients were assigned to deterioration group or non-deterioration (stable or improved) group. The clinical characteristics of the 2 groups were compared. The factors influencing the deterioration of nutritional status were screened by logistic regression analysis. The correlation between nutritional status deterioration and adverse reactions (radiation esophagitis, pulmonary infection, neutropenia, thrombocytopenia, and elevated aminotransferase) was analyzed by Spearman correlation analysis. Results There were no significant differences in gender, radiotherapy dose, initial nutritional status, or performance status score between the 2 groups for the deterioration of nutritional status after radiotherapy (all P＞0.05). The proportions of patients with previous surgical history of esophageal cancer, synchronous chemoradiotherapy, initiation of radiotherapy at less than 90% of target calorie requirement, and clinical stage Ⅳ were significantly higher in the deterioration group than those in the non-deterioration group (all P＜0.05). Logistic regression analysis showed that clinical stage Ⅳ (odds ratio ［OR］＝4.684, 95% confidence interval ［CI］ 1.252-17.519, P＝0.022) and previous surgical history of esophageal cancer (OR＝7.338, 95% CI 1.878-28.666, P＝0.004) were the independent adverse risk factors for the deterioration of nutritional status after radiotherapy/radiochemotherapy. The timing of radiotherapy intervention was also an independent risk factor for the deterioration of nutritional status, and taking the tolerance of 70%-90% target energy as the reference level, starting radiotherapy when the tolerance of 90%-100% target energy had the optimal protection of nutritional status (OR＝0.166, 95% CI 0.050-0.551, P＝0.003). Spearman correlation analysis showed that the deterioration of nutritional status was positively correlated with elevated transaminases after radiotherapy (r_s＝0.283, P＝0.003), while it was not correlated with the other adverse reactions (all P＞0.05). Conclusion Under the standard nutritional intervention model, patients with previous surgery and recurrent metastatic esophageal cancer who receive radiotherapy/chemoradiotherapy are still at risk of nutritional status deterioration. Tolerance to 90%-100% target energy requirement may be a more appropriate timing for radiotherapy intervention. When the nutritional status deteriorates during treatment, it is necessary to be alert to the elevated transaminases.