The invention provides a preparation method and application of radix astragali 
saponin. The preparation method of radix astragali 
saponin comprises the following steps: (1) adding water to radix astragali, extracting by decoction, filtering, and concentrating the filtrate; (2) adding 
ethanol to precipitate, and standing; (3) recovering the 
ethanol from the supernatant, diluting with water, injecting into macroporous adsorbent resin columns, and sequentially eluting with water, a 20% (volume / volume) 
ethanol water solution and a 70% (volume / volume) ethanol water solution; and (4) removing the ethanol in the eluent of the 70% (volume / volume) ethanol water solution, and 
drying to obtain the radix astragali 
saponin. The research indicates that the radix astragali saponin can activate the vascular endothelial 
cell NO-cGMP 
system, inhibit the 
calcium influx of 
vascular smooth muscle cells, and open the K[Ca] and K[ATP] channels, thereby relaxing the smooth muscles, especially vascular smoothmuscles; and therefore, the radix astragali saponin has an obvious function of vasodilatation, and is suitable for preventing and treating cardiovascular diseases, such as lowering the 
blood pressure, resisting 
angina pectoris and the like.