Abstract: Measuring ICF neutron yield by Pb activation is performed in the target chamber, but calibrating the Pb detect system using an accelerator neutron source is not performed in the target chamber. The difference between the measuring environment and the calibrating environment will affect the measurement accuracy, for the neutrons scattered by the target chamber will contribute to the production of radionuclide 207mPb which emits detectable γ radiations.In this paper, by Monte Carlo method, the effect on the measurement accuracy has been calculated and the result is less than 1‰.This effect is defined as the ratio of the number of 207mPb(Ns) generated from the target chamberscattered neutron activation to the number of 207mPb(N) generated without target chamber existing. The target chambe