Objective To evaluate the effects of intestinal barrier function indicators (diamine oxidase ［DAO］, D-lactic acid ［DLA］, and endotoxin ［ET］) on common adverse reactions and nutritional status of patients with gastrointestinal tumor after chemotherapy, and to determine the best predictive values of intestinal barrier function indicators for them. Methods A total of 77 patients with gastrointestinal tumors who were admitted to Anqing First People’s Hospital of Anhui Medical University from Sep. 2019 to May 2022 were enrolled. DAO, DLA and ET of patients before chemotherapy and pre- and post-chemotherapy patient generated subjective global assessment (PG-SGA) scores were collected. The effects of intestinal barrier function on adverse reactions and nutritional status after chemotherapy were analyzed by comparing the levels of DAO, DLA and ET in patients with different clinical characteristics. The influencing factors of adverse reactions and nutritional deterioration were screened using binary logistic regression analysis. The best predictive values of DAO, DLA and ET for adverse reactions and nutritional deterioration were determined using receiver operating characteristic curve and Youden index. Results The occurrence of myelosuppression was not affected by the levels of DAO, DLA and ET. Patients with vomiting, diarrhea or nutritional deterioration after chemotherapy had significantly higher DAO and DLA before chemotherapy (all P＜0.05). Patients with diarrhea or elevated transaminase after chemotherapy had significantly higher ET before chemotherapy (both P＜0.001). Regression analysis showed that DAO and DLA before chemotherapy and gastrointestinal surgery within 3 months were related to the occurrence of vomiting (all P＜0.05). DLA is an independent influence factor of diarrhea after chemotherapy (P＝0.001). Late cycle chemotherapy and elevated DLA were the influencing factors of nutritional deterioration after chemotherapy (both P＜0.05). The best predictive value of ET for transaminase elevation after chemotherapy was 17.36 U/L (area under curve ［AUC］ ＝0.850, P＜0.001). The best predictive values of DAO and DLA for vomiting were 10.47 U/L (AUC＝0.727, P＝0.001) and 18.55 mg/L (AUC＝0.708, P＝0.003), respectively. The best predictive value of DLA for diarrhea was 17.63 mg/L (AUC＝0.920, P＜0.001). The best predictive value of DLA for nutritional deterioration was 12.77 mg/L (AUC＝0.684, P＝0.006). Conclusion Intestinal barrier function indicators (DAO, DLA, and ET) have predictive value for vomiting, diarrhea, elevated transaminase and nutritional deterioration after chemotherapy.