Multi foil activation method is widely used in reactor pressure vessel neutron fluence and neutron spectrum measurement. Usually, several sets of dosimeters are installed in different locations in reactor vessel surveillance capsule to monitor local neutron fluence and neutron spectrum. The dosimeter set is composed of several dosimeters with different threshold activation energy to obtain neutron fluence at different energy region. For some accidental occasions, dosimeters may be missing or lose their function that can’t provide effective measurement data. In these cases, analysis must be implemented to assess the impact on the neutron fluence and neutron spectrum measurement results. This article analyzed the impact on neutron fluence and neutron spectrum when an Iron foil was missing in a reactor vessel surveillance capsule. Several comparisons were carried out during the analysis, including comparisons of spectrum adjustment calculation results with and without iron foil using measured data in a testing reactor, comparisons of spectrum adjustment calculation results with and without iron foil using another dosimeter set in the same surveillance capsule. The analysis results showed that the missing of iron foil had very little influence on neutron fluence and neutron spectrum results in the two cases. However, the analysis in this article was not comprehensive enough as several facts were not considered, such as initial neutron spectrum, foil reaction rate uncertainties used in spectrum adjustment and so on. Therefore, further detailed analysis may be implemented to find out whether the impact of missing foil can be ignored or not on neutron fluence and neutron spectrum measurement.