Objective To analyze the influencing factors of malnutrition risk in breast cancer patients undergoing chemotherapy and construct a nomogram model based on the analysis results. Method The relevant data of 231 breast cancer patients undergoing chemotherapy in Affiliated Hospital of Yangzhou University were retrospectively analyzed, taking body mass index (BMI) <18.5 kg/m2 and nutritional risk screening 2002 (NRS 2002) score≥3 points as the criteria for judging malnutrition, they are divided into malnutrition group (65 cases) and normal nutrition group (166 cases). The multivariate Logistic regression were used to analyze the influencing factors of malnutrition risk in breast cancer patients undergoing chemotherapy, and a nomogram prediction model was constructed based on the analysis results, then use the area under the receiver operating characteristic (ROC) curve and the H⁃L fit curve to evaluate the effectiveness and discrimination of the model. Use the decision curve analysis (DCA) to evaluate the predictive value of the model. Result 65 patients in this study developed malnutrition, accounting for 28.14%. There were significant differences in age, adjuvant radiotherapy, liver damage, relative dose intensity of chemotherapy drugs, oral mucositis, gastrointestinal reactions (constipation and abdominal distension) between the two groups (P<0.05). Multivariate Logistic regression analysis showed that age ≥60 years old, adjuvant radiotherapy, relative dose intensity of chemotherapeutics of 85%-100%, and gastrointestinal reactions were independent risk factors for malnutrition in patients undergoing chemotherapy after breast cancer surgery (P<0.05). According to the results of multivariate Logistic regression analysis, a nomogram prediction model for malnutrition in patients undergoing chemotherapy after breast cancer surgery was constructed and evaluated, the area under the ROC curve was 0.824, the sensitivity was 73.8%, and the specificity was 96.0%. The H⁃L fit curve results showed that χ2=7.241, P=0.412. Conclusion Age≥60 years old, adjuvant radiotherapy, the relative dose intensity of chemotherapy drugs of 85%-100% and gastrointestinal reactions were independent risk factors for malnutrition in patients undergoing chemotherapy after breast cancer surgery. The constructed nomogram prediction model has good predictive value, and can be used as a tool to screen the risk of malnutrition in patients undergoing chemotherapy after breast cancer surgery