The present invention provided a continuous flow process for the synthesis of phenylhydrazine salts and substituted phenylhydrazine salts. Diazotization, reduction, acidic hydrolysis and salifying with acids are innovatively integrated together. Using acidic liquids of aniline or substituted aniline, diazotization reagents, reductants and acids as raw materials, phenylhydrazine derivative salts is obtained through the synthesis process, which is a three-step continuous tandem reaction including diazotization, reduction, acidic hydrolysis and salifying. The described synthesis process is a kind of integrated solutions, which is carried out in an integrated reactor. The feed inlets of the integrated reactor are continuously filled with raw materials. In the integrated reactor, diazotization, reduction, acidic hydrolysis and salifying are carried out continuously and orderly, and phenylhydrazine salts or substituted phenylhydrazine salts is obtained in the outlet of the integrated reactor without interruption. The total reaction time is no more than 20 min.