The invention provides a new process for synthesizing high-purity p-phenylenediamine through a continuous method. The process comprises: taking p-nitroaniline and hydrogen as raw materials, taking water as a solvent, in a fixed bed reactor or shell-and-tube reactor and in the presence of a catalyst, carrying out a hydrogenation reaction to prepare a p-phenylenediamine crude product, performing separation and purification to the crude product to obtain pure p-phenylenediamine, and recycling water and hydrogen for cyclic application. The process allows the p-nitroaniline conversion rate to reach100%. The process routine is simple and is easy to control; the catalyst is a supported nickel-based catalyst, is low in cost, and doesn't pollute the environment; the solvent is methanol or ethanoland is low in price; and the raw materials are available. The purity of p-phenylenediamine is extremely high and can reach 99.9%.