Investigations have shown that postmenopausal women with type II diabetes have a 20% increased risk of breast cancer compared to the general population, a statistically significant difference, and a significantly increased risk of distant metastases and of death from disease found at 5 years of follow-up. Studies have shown that breast cancer patients with combined type II diabetes are older, have a higher proportion of postmenopausal patients, and have lower disease-free and overall survival rates and worse prognosis in the diabetic group than in the group of patients without diabetes, with a nearly 15% decrease in 5-year survival. The specific mechanism by which diabetes promotes breast cancer is unclear, and the available findings suggest that it is related to several aspects: including hyperinsulinemia, body mass index, insulin-like growth factor-1, and hyperglycemia-induced oxidative stress and ER status. Insulin promotes the development and progression of breast cancer, and metformin improves the prognosis of breast cancer patients with type II diabetes; being overweight or obese significantly increases the prevalence of breast cancer; and postmenopausal ER(-) breast cancer with diabetes has a poorer prognosis and a significantly higher rate of distant metastasis. Studies have shown that tumors are more aggressive in hyperinsulinemic breast cancer patients, and there is a direct relationship between fasting insulin, tumor recurrence, and death; researchers have found that breast cancer patients with combined diabetes have higher pathological tumor grade, greater body mass index, and larger primary tumor diameter; and that breast cancer progression can be more rapid than in non-diabetic patients, even at the same time of tumor development. Diabetic patients have a 14% to 21% increased likelihood of tumor progression and a 16% increased risk of tumor spread to the lymph nodes. For female patients with a history of diabetes, those aged 45 years or older have a very high chance of developing breast cancer and belong to the high-risk female population. They should pay attention to regular annual breast tumor screening to achieve early detection and early treatment, and mammography is recommended.