The invention discloses a supplementary cooling device for a spent fuel pool of a nuclear power plant. The device comprises a first loop which is positioned in a nuclear island plant building, and a second loop which is positioned outside the nuclear island plant building, wherein water in the spent fuel pool can circularly flow in the first loop; the heat is conveyed to the second loop through the first loop by utilizing an intermediate heat exchanger; heat of the fluid in the second loop is dissipated through a cooling tower. Compared with the prior art, the device has the advantages that adouble-loop cold chain is designed to limit the spent fuel pool into the nuclear island plant building, so that radioactive substances can be prevented from leaking from the nuclear island plant building by accident, and as a result, the safety and the reliability of the device are improved, and the cooling capacity of the spent fuel pool can be obviously improved; meanwhile, a double-die power supplying manner is adopted, and an independent hot trap is additionally arranged, so that the emergency treatment capacity of the spent fuel pool to whole-plant power-off accident and hot trap losing accident can be improved.