The invention discloses a method for separating boron and radionuclides in radioactive wastewater by using a long flow passage. The method comprises the following steps: (1) setting a membrane stack, alternately arranging a plurality of anodes and cathodes in the membrane stack, and setting a cation exchange membrane and an anion exchange membrane between the anodes and the cathodes to form cathode chambers, anode chambers and fresh water chambers, wherein the adjacent anodes and cathodes form an electric field; (2) filling ion exchange resin into the anode chambers, the cathode chambers and the fresh water chambers; and (3) connecting the cathode chambers, the anode chambers and the fresh water chambers in series respectively to form three flow passages, and introducing the radioactive wastewater into the three flow passages, wherein borate ions in the fresh water chambers are migrated into the anode chambers under the action of the electric field and nuclide ions in the fresh water chambers are migrated into the cathode chambers, thereby separating boron and radionuclides in the radioactive wastewater in the fresh water chambers; and during separation, the ratio of the mean concentrations of boron in the anode chambers and the fresh water chambers in the same electric field is controlled to be not more than 35.