The invention discloses a method for preparing salvianolic acid A. The method includes the following steps that firstly, salvianolic acid B is prepared into a solution with the concentration of 35-45 mg / mL by means of NaOH or NaHCO3 with the pH value of 3.5-4.5, the solution is placed in a subcritical water reaction kettle, after the temperature of a heating furnace reaches 170-190 DEG C and is stabilized, the reaction kettle is placed into the heating furnace, the reaction kettle is taken out after 50-70 min and placed in ice water bath or cold water to be cooled, the liquid is taken out and subjected to freeze-drying, and a crude product rich in salvianolic acid A is obtained; secondly, salvianolic acid A is separated and purified by means of high-speed countercurrent chromatography, wherein a solvent system is prepared from petroleum ether, ethyl acetate, n-butyl alcohol and water according to the ratio of 2:3:1:9, 10 mM of trifluoroacetic acid is added to an upper phase to form a stationary phase, 10 mM ammonia water is a lower phase and serves as a mobile phase, the volume of a high-speed countercurrent chromatography column is 200-400 mL, the sample loading amount is 1.0-1.2 g, the rotation speed is 600-1000 rpm, the flow speed is 1-4 mL / min, and the detection wavelength is 280 nm. The method is low in cost, easy to operate and high in efficiency, salvianolic acid crude extracts can be converted on a large scale, and a salvianolic acid A monomeric compound with the purity higher than 98% is separated and prepared.