The invention belongs to the technical field of nitrile group hydrolysis and demethylation hydrolysis reaction which is one of the key steps for chemical synthesis of a methyldopa drug or a chiral drug of L-methyldopa [L-alpha-methyl-(3,4-dimethoxyphenyl)-alpha-aminopropionitrile, the same below], that is, a reaction for preparing alpha-methyl-(3,4-dimethoxyphenyl)-alpha-aminopropionitrile or L-alpha-methyl-(3,4-dimethoxyphenyl)-alpha-aminopropionitrile by nitrile group hydrolysis and demethylation hydrolysis. Under an acidic condition, the preparation of methyldopa by nitrile group hydrolysis and demethylation hydrolysis of alpha-methyl-(3,4-dimethoxyphenyl)-alpha-aminopropionitrile (or hydrochloride) requires a long period, a large material using amount, and heating hydrolysis of high-concentration acids (and the long-time heating of the high-concentration acids allows the acids to volatilize, so continuous acid addition is required during hydrolysis); however, the long-time high-concentration acid heating hydrolysis can cause unknown change of certain compound structures of alpha-methyl-(3,4-dimethoxyphenyl)-alpha-aminopropionitrile (or hydrochloride), and thus by-products are increased, and the purity and yield are decreased. Therefore, improvement of the hydrolysis technology is necessary.