Abstract: Erbium(Er) is a kind of long-term burnable poison which is suitable for light water reactor(LWR). The irradiation of Er₂O₃ is a basic means to study the performance of burnable absorber Er in the reactor. Er₂O₃ samples were irradiated in the irradiation channel (neutron fluence rate is about 2×10¹⁴/(cm²•s)) of high flux engineering test reactor(HFETR) for 91.3 h. The Er isotopic abundances of irradiated samples were measured by thermal ionization mass spectrometry(TIMS). The peak jumping mode was applied to determine the 6 isotopes of Er, and the low abundance ¹⁶²Er was detected by the Faraday cup. Isobaric interferences can be reduced by accurate control of heating current. Experiment shows that the isobaric interferences of ¹⁶⁸Yb, ¹⁷⁰Yb were reduced when the evaporation filament current was below 1 400 mA and ionization filament current was below 5500 mA. The measurement results show that the ¹⁶⁶Er，¹⁶⁷Er and ¹⁶⁸Er isotopic abundance changes greatly after neutron irradiation, and the variation of abundance is closely related to the neutron absorption cross section.