A large number of clinical cases have been observed according to evidence-based principles in medicine, and it has been demonstrated that postoperative chemotherapy for invasive breast cancer patients can significantly improve survival. As we know, breast cancer is one of the most effective tumors among solid tumors in terms of chemotherapy application, and chemotherapy plays an important role in the whole treatment process. Surgery removes as much of the tumor load as possible, and the remaining tumor cells can be easily killed by the chemotherapy anti-cancer drugs administered after surgery. However, there is a certain theoretical basis as to why 6 times are appropriate. 1.Even if the chemotherapy drugs can kill 99.99% of the residual cancer cells, there will still be 108 cancer cells that will recur or metastasize in the clinic. 2. The killing of cancer cells is logarithmically decreasing, and it is impossible to kill them all in one or two times. 3.The proliferation cycle of cancer cells is long, so it is impossible to kill all of them in one or two times. 4. Infantile cancer cells are not sensitive to chemotherapy and can only be killed by chemotherapy drugs when they enter the proliferating stage. Therefore, for breast cancer patients, adjuvant chemotherapy should be used early after surgery and 6 courses of treatment are appropriate to kill subclinical metastases. If the duration is too long, it will increase the damage to human immunity, and if it is too short, it will lead to premature recurrence or metastasis too soon.