This invention relates to a novel soy isoflavone product with high purities and strong biological activities and the method of producing the same from natural soybeans, soybean materials (i.e. tofu dregs, soy molasses) and other plant sources. The method includes three steps consisting of extraction with an organic solvent, hydrolysis using an acid and crystallization using an antisolvent. The procedure is very simple and thus can be easily adapted for large-scale manufacturing. Moreover, the procedure is able to produce a high yield of total isoflavones at a lower cost. HPLC analysis and E-Screen bioassay reveal that the obtained product not only contains a high content of isoflavone aglycones by weight of dry matter but also exhibits strong estrogenic activity toward human cells. Therefore, the product should be efficacious for relieving menopausal symptoms and other estrogen-deficient diseases and can be used in health care supplements or as additives for foods, beverages or cosmetics.