The invention relates to a purification device for treating radioactive wastewater. The device comprises a cross-flow membrane filter, a first water supply pipe, a purified water pipe and a concentrated water pipe. The cross-flow membrane filter comprises a filtering cavity and a filter membrane arranged in the filtering cavity, the filtering cavity is provided with a water inlet end, a concentrated water outlet end and a purified water outlet end, the water inlet end and the concentrated water outlet end are positioned on the water inlet side of the filter membrane, and the purified water outlet end is positioned on the water outlet side of the filter membrane; the first water supply pipe is used for transmitting radioactive wastewater, and the water outlet end of the first water supply pipe communicates with the water inlet end of the filtering chamber; the purified water pipe is used for transmitting water filtered by the filter membrane, and the water inlet end of the purified water pipe communicates with the purified water outlet end of the filtering chamber; and the concentrated water pipe is used for discharging water intercepted by the filter membrane, the water inlet end of the concentrated water pipe communicates with the concentrated water outlet end of the filter chamber, and the concentrated water pipe is provided with a water flow regulating valve. The purification device for treating radioactive wastewater has a simple structure, and can improve the water purification efficiency of radioactive wastewater.