Abstract： Objective To study influencing factors which cause the endometrial diseases in patients with breast cancer after operation. Methods A retrospective study was performed on 212 breast cancer post-operation patients with endometrial diseases between June 2006 and January 2018 in Women's Hospital School of Medicine Zhejiang University to analyse the factors which influenced the endometrial diseases. Results The abnormal uterine bleeding and endometrial thickness were related to the severity of endometrial disease in patients with breast cancer, and they were independent risk factors for breast cancer patients to have endometrial cancer (P<0.05). When the diagnostic cut off value of endometrial thickness was ≥0.49 cm, the sensitivity and specificity to endometrial cancer were 78% and 25%, respectively. The average endometrial thickness was (0.56 ± 0.39) cm in patients who were treated by selective estrogen receptor modulator (SERM) after gynecological surgery, which was significantly thicker than that of aromatase inhibitor (AI) group [(0.33 ± 0.23) cm] and no treatment group [(0.44 ± 0.28) cm, P<0.05]. The endometrial disease recurrent rate and reoperation rate in SERM group were (26.2%, 14.3%) slightly higher than that of AI group (9.5%, 4.8%) and no treatment group (21.6%, 4.9%), but there were not significant differences (all P>0.05). Conclusions The clinical symptom of abnormal uterine bleeding and thickening endometrium are risk factors for breast cancer patients to have endometrial cancer. The endometrial thickness has high predictive value for breast cancer patients to diagnose endometrial cancer. The SERM treatment increases the endometrial thickness, recurrent rate and reoperation rate in post-operation patients.