A migration prevention system for radioactive wastewater from an underground nuclear power plant. The underground nuclear power plant includes a nuclear island including an underground cavern group including a reactor cavity and auxiliary cavities. The migration prevention system includes a protective layer coating the reactor cavity and an impermeable layer surrounding the nuclear island. The protective layer includes an inner liner, a drainage layer, and a filling layer of rock fractures in that order. The inner liner is configured to prevent exosmosis of the radioactive wastewater of the reactor cavity. The drainage layer is configured to gather and drain seepage water. The impermeable layer is disposed in the periphery of the underground cavern group including the reactor cavity and the auxiliary cavities, and is configured to isolate the underground cavern group from natural underground water.