Abstract: The neutron source is used to increase the neutron fluence of reactor in subcritical condition. In the actual operation, either neutron source decay caused by long-time shutdown or neutron source damage could make further use impossible. By calculating and analyzing spontaneous neutron source of used fuel assembly and source range detector response, the possibility of substituting used fuel assembly for neutron source was discussed. According to the result, the requirement of neutron count rate monitoring can be satisfied on condition that the first loading fuel assembly has a burnup of more than 24 100 MW•d•tU^-1. This method can provide a solution to unexpected neutron source damage.