Abstract: When mono-crystalline silicon moving into HFETR core through the irradiation channel, the reactivity disturbance is introduced and the local neutron flux is affected. The mathematical calculation model of HFETR is established through the combination of Monte Carlo N–Particle code（MCNP5） and Monte Carlo N–Particle extended code（MCNPX2.6）, and the availability of the model are verified by the reactor physical start-up experiment. The reactivity disturbance introduced by different quality of mono-crystalline silicon moving into HFETR core from 8^# channel are calculated, and the effects of local neutron flux in neighboring ionization chamber channel when 8 kg mono-crystalline silicon moving into HFETR core from 8# channel are analyzed. The results show that the reactivity disturbance introduced by mono-crystalline silicon moving into core is small and meets the safety requirements of HFETR, however, it will lead to the distortion of neutron flux distribution in adjacent channel to affect neutron detector.