Abstract: In order to optimize the extraction process of isoflavones from Korean soybean paste and study the anti-tumor activity of isoflavones in Korean soybean paste,this experiment used ultrasonic method to extract isoflavones from Korean soybean paste,which was optimized by single factor experiment and orthogonal experiment. The extraction process was carried out,and the proliferation inhibition of normal breast cells,MDA-MB-231 and MCF-7 breast cancer cells was determined by CCK-8 method. Western Blot was used to detect the expression of apoptosis-related proteins in two breast cancer cells treated with isoflavone of Korean soybean paste. The optimum extraction conditions were determined by single factor and orthogonal experiments. The extraction time was 80 min,the ratio of material to liquid was 1:24,and the temperature was 50℃. The yield was the highest at 0.802%. The results of CCK-8 showed that different concentrations of Korean soybean paste isoflavone had no toxic side effect on normal cells. The proliferation inhibition of MDA-MB-231 and MCF-7 breast cancer cells was dose-dependent and there were significant differences (p<0.05). The detection results of apoptosis-related proteins in two kinds of cells treated with different concentrations of Korean soybean paste isoflavone showed that the two kinds of cells increased with the concentration of isoflavone,the expression of Bax and caspase-3 significantly increased compared with the control group (p<0.05),while the expression of anti-apoptotic protein Bcl-2 significantly decreased (p<0.05). With dose dependence. It indicates that Korean soybean paste isoflavone can inhibit the growth of breast cancer cells.