Objectives A high-performance liquid chromatography (HPLC) method for detecting soybean extracts in feeds was established to provide support for the rational use of soy isoflavones in animal husbandry. Methods By reviewing literature and using HPLC chromatography technology, the sample was extracted by ultrasound from methanol solution and eluted with acetonitrile 0.1% acetic acid-water as the mobile phase gradient. The detection was performed at a wavelength of 260 nm. Results The detection method established with HPLC had a linear range of 0.2-20 μg/mL, with a quantification limit of 0.197 μg/mL. The mass concentration of 2 μg/mL and 15 μg/mL was added for recovery, with a recovery rate of 97.45%-102.54%. Conclusions According to the established HPLC method for detecting 5% purity of soy isoflavones, the content of six components including daidzin 1.13%, daidzin 0.22%, genistein 2.79%, daidzein 0.55%, daidzin 0.06%, and genistein 0.29% were detected. The total flavonoid content was 5.04%.