Objective To analyze the influencing factors of demoralization syndrome in breast cancer patients, and to provide evidence for clinical nursing intervention. Methods Totally, 327 inpatients with breast cancer were surveyed with the Demoralization Scale, the Self-Perceived Burden Scale and the social support Scale. Results Of the 327 patients, 152 cases (46.48%) had demoralization syndrome. Multivariate logistic regression analysis showed that higher age, urban residence and higher social support were protective factors for demoralization syndrome in breast cancer patients, while cancer stage Ⅲ-Ⅳ, severe pain and numbness, upper limb lymphedema and heavier self-perceived burden were risk factors (P<0.05, P<0.01). Conclusion A large share of breast cancer patients developed demoralization syndrome, which were influenced by multiple factors. Targeted measures should be taken to improve mental health of breast cancer patients.