Background: With the significant improvement of the cure rate and survival rate of cancer patients, the survivors face quality-of-life problems, such as a significant decline in reproductive system development, ovarian reserves and function, and even fertility loss and early menopause. These problems are often highly associated with chemotherapy-induced ovarian damage in cancer treatment. However, there are no ideal treatment strategies at present. In our attempt to develop reagents and approaches for delaying ovarian aging and protecting che-motherapy-induced ovarian injury, we recently found that metformin may be the most promising drug to protect female malignant tumor patients from chemotherapy-induced ovarian injury. This trial aims to test whether administration of metformin during chemotherapy can protect the normal ovarian function of patients with early breast cancer. Methods: This study is prospective, randomized, double-blind and placebo-controlled. Female patients with early breast cancer (N = 314) will be randomly assigned to two groups (placebo, metformin 2000 mg). Metformin will be administered during and after chemotherapy for patients with stage I-IIIa breast cancer. The primary outcome will be the menstruation recovery rate 12 months after chemotherapy, defined as recovery of menstruation twice in a row within 1 year. Patients will be followed up for 5 years to observe long-term ovarian function and prognosis, such as overall survival (OS), objective response rate (ORR), and disease-free survival (DFS). Quality of life and safety will also be assessed. Discussion: Our research will provide a new treatment strategy for fertility protection, and clinical treatment guidance for cancer patients.