NOVELTY - Preparation of (6S)-5 methyl-tetrahydrofolic acid quaternary ammonium salt involves suspending folic acid in 5-7 times of water, adding 1N-2N aqueous alkali solution under protection of nitrogen, adding water, keeping reaction for 0.5-1.5 hours at 60-75 degrees C, decoloring by active carbon, filtering, adding antioxidant, 2N-3N of acid to adjust pH value to 2.5-3.5, filtering, washing solid, vacuum drying to obtain tetrahydrofolic acid; adding suspension to 5-7 times amount of water, adding alcohol water solution, adding reducing agent for 45 minutes, keeping the reaction to 1-1.5 hours.    USE - For preparation of (6S)-5 methyl-tetrahydrofolic acid quaternary ammonium salt; for synthesis of zofenopril calcium salt (claimed) used as medicine or food additive.    ADVANTAGE - The method allows preparing folic acid salt with high stability, uses mild reaction condition, using easy to obtain raw material reactant such as potassium borohydride, sodium bisulfite, which greatly reduces the production cost, reduces the environmental pollution and is suitable for industrial production.    DETAILED DESCRIPTION - Preparation of (6S)-5 methyl-tetrahydrofolic acid quaternary ammonium salt involves suspending folic acid in 5-7 times of water, adding 1N-2N aqueous alkali solution under the protection of nitrogen, adding water, keeping the reaction for 0.5-1.5 hours at 60-75 degrees C, decoloring by active carbon, filtering, adding antioxidant, 2N-3N of acid to adjust the pH value to be 2.5-3.5, filtering, washing solid, vacuum drying to obtain the tetrahydrofolic acid; adding the suspension to 5-7 times amount of water, adding alcohol water solution under the protection of nitrogen for 35-40% under 35-45 degrees C, adding reducing agent for 45 minutes, keeping the reaction to 1-1.5 hours, after the reaction with 3N to 5N acetic acid to pH 6-7, adding 8-11% of calcium chloride solution, filtering, concentrating the filtrate and adding calcium chloride solution at room temperature, stirring at room temperature, filtering, decoloring, adding alcohol solution with the concentration of 5-7 times amount, filtering, vacuum drying to obtain the 5-methyl tetrahydrofolic acid, reacting 5-methyl tetrahydrofolic acid with organic alkali, then adding absolute ethyl alcohol to reflow for 0.3-0.6 hours, cooling to room temperature, filtering, recrystallizing the filter cake with alcohol, re-crystallizing the solid by adding water, using acid to regulate the pH to 2, reducing the temperature to 5 degrees C, filtering, washing into neutrality by water, vacuum drying to obtain the (6S)-5-methyl tetrahydro folic acid salt.