Estrogen is considered to have an important role in the development of the breast. Blocking the action of estrogen, including inhibiting estrogenic activity and lowering estrogen levels, is widely used in the adjuvant treatment of breast cancer. Soybeans are rich in phytoestrogens, which are mainly isoflavones. Soy isoflavones are flavonoids, a class of secondary metabolites formed during the growth of soybeans, which are bioactive substances. Because it is extracted from plants and has a similar structure to estrogen, soy isoflavones are also known as phytoestrogens. It is a natural estrogen receptor modulator, which has both estrogen-like and anti-estrogenic effects. Epidemiological studies have shown that Asian countries with low incidence of breast cancer such as China and Japan have high consumption of soy products. Phytoestrogens are converted to estrogens in the intestine, but they may have weak estrogenic but also anti-estrogenic effects. Phytoestrogens do not correlate with the risk of breast cancer, rather some studies have shown that phytoestrogen intake is associated with a reduced risk of breast cancer and may have an important role in breast cancer prevention. What effect will phytoestrogens have on breast cancer patients. Several studies have found that the intake of soy products reduces the risk of breast cancer recurrence and death. Especially for Asian women. Therefore, soy does not increase the risk of breast cancer, but instead may have its important role in breast cancer prevention. For breast cancer patients, soy can reduce the risk of recurrence and death. But this is not to say that soy is a panacea, as many people may still remember the days when green beans cured all diseases. Soy itself also contains substances that are not good for health. It may inhibit the action of trypsin, which affects the absorption of protein. The processing of soybeans may allow soy products to contain high levels of aluminum, which may be harmful to our health. In addition it may interfere with the absorption of calcium, magnesium, copper and especially zinc. Soy protein may cause enlargement of the pancreas and thyroid gland, and soy is not suitable for patients with hypothyroidism.