NOVELTY - The method comprises adding ammonia, water, veratone, ammonium chloride and sodium cyanide to a reactor, filtering after reaction to obtain mother liquor for synthesizing DL-aminopropionitrile, resolving the DL-aminopropionitrile under the effect of resolving agent and then filtering after reaction to obtain D-aminopropionitrile, adding extracting solvent to the filtrate, adding acid to an extraction layer and then filtering to obtain L-aminopropionitrile, and subjecting ammonia, water and D-aminopropionitrile to racemization reaction and then filtering.    USE - The method is useful for preparing L-methyldopa.    ADVANTAGE - The method is capable of economically preparing the L-methyldopa in an environmentally friendly manner.    DETAILED DESCRIPTION - The method comprises adding ammonia, water, veratone, ammonium chloride and sodium cyanide to a reactor, filtering after reaction to obtain mother liquor for synthesizing DL-aminopropionitrile, resolving the DL-aminopropionitrile under the effect of resolving agent and then filtering after reaction to obtain D-aminopropionitrile, adding extracting solvent to the filtrate, adding acid to an extraction layer and then filtering to obtain L-aminopropionitrile, subjecting ammonia, water and D-aminopropionitrile to racemization reaction and then filtering to obtain D-aminopropionitrile racemized mother liquor, hydrolyzing L-aminopropionitrile to obtain L-methyldopa, mixing the DL-aminopropionitrile synthesized mother liquor or D-aminopropionitrile racemized mother liquor with organic solvent and then heating to 40-110 degrees C and reacting for 1-24 hours, standing and delaminating after reaction to obtain the first water layer and the first organic layer, where the organic solvent is non-solvent of water and good solvent of veratone, or adding base and water to the wastewater resolved by DL-aminopropionitrile, regulating the pH to 9-11, heating the mixture to 40-110 degrees C and then reacting for 1-24 hours, standing and delaminating after reaction to obtain the second water layer and the second organic layer or mixing the DL-aminopropionitrile synthesized mother liquor or D-aminopropionitrile racemized mother liquor with the DL-aminopropionitrile resolved waste liquor, heating the mixture to 40-110 degrees C and then reacting for 1-24 hours, standing and delaminating after reaction to obtain the third water layer and the third organic layer, recovering the solvent from the obtained first organic layer, the second organic layer or the third organic layer, and high vacuum distilling the mixture at pressure not higher than 5 kPa to obtain veratone and recovering. A volume ratio of DL-aminopropionitrile synthesized mother liquor or D-aminopropionitrile racemized mother liquor is 2-6:1. A volume ratio of DL-aminopropionitrile resolved waste liquor to water is 2-6:1. A volume ratio of DL-aminopropionitrile synthesized mother liquor or D-aminopropionitrile racemized mother liquor to DL-aminopropionitrile resolved waste liquor is 2-4:1. The method further comprises aerating the first water layer for 3-6 hours, aerating the second water layer for 3-6 hours, aerating the third water layer for 3-6 hours, mixing the DL-aminopropionitrile synthesized mother liquor or D-aminopropionitrile racemized mother liquor with the organic solvent, heating to backflow temperature and reacting for 5-12 hours, adding base and water to the DL-aminopropionitrile resolved waste liquor, regulating pH to 7-14, heating to backflow temperature and then reacting for 5-12 hours, mixing DL-aminopropionitrile synthesized mother liquor or D-aminopropionitrile racemized mother liquor with the DL-aminopropionitrile resolved waste liquor, and heating to backflow temperature and then reacting for 5-12 hours. The high vacuum distilling pressure is less than 0.5 kPa. The high vacuum distilling step is performed at 112-120 degrees C.