The invention relates to a method for treating radioactive waste liquid in a nuclear power plant through electrochemical synthesis of ferrite. The method comprises the following steps: adding supporting electrolyte into the radioactive waste liquid in the nuclear power plant, and uniformly stirring to obtain a mixed solution; adding an alkaline solution into the mixed solution to adjust the pH, inserting electrodes, performing constant-temperature electrolytic synthesis, standing the mixed solution obtained after completion of a reaction, filtering, performing solid-and-liquid separation to purify the radioactive waste liquid in the nuclear power plant, and efficiently removing a precipitate through magnetic separation. By the method, through electrochemical synthesis of the ferrite, the ferrite is generated in the electrolytic process, soluble metal ions in the solution and the ferrite can form a stable ferrite coprecipitate and the ferrite coprecipitate is removed, so that radioactive element ions in the radioactive waste liquid in the nuclear power plant are efficiently removed; the method has the advantages of a simple process, convenient operation, unlikeliness in secondary pollution and the like.