Alumina-forming duplex FeNiCrAl alloy with superior mechanical properties and high-temperature oxidation resistance has been regarded as an accident-tolerant fuel cladding material. This work investigated the effect of aging and the radiation resistance of FeNiCrAl duplex alloy after up to 2000 hours thermal aging and 3.5 MeV Fe13+ ion radiation. The ferrite phase containing B2 phase shows much fewer radiation defects and lower radiation hardening rate than conventional FeCrAl alloy. The coherent B2 phase precipitate is critical in hindering dislocation and inhibiting the formation of radiation induced defects which is proved via experiment and molecular dynamics simulation. Besides, the Al depletion of B2 phase and nano-sized precipitation are characterized by using Time-of-Flight Secondary Ion Mass Spectrometry and Atom Probe Tomography. These findings effectively illuminate microstructure evolution of FeNiCrAl duplex alloy after long-term aging and radiation and lead to new insight for designing advanced accident-tolerant fuel material.