Abstract: In the course of the core flux density measurement, detector foil and its backing materials are introduced into the gap between slab-type fuel element. Thus the moderale medium is changed and as well as the moderation and absoration of neutrons. It brings error in measurement results, which should been considered. In this paper, the disturbance to the neutron field caused by the appearance of different backing materials is studied in detail with Monte-Carlo methods, which helps us in choosing the backing material reasonably and the data analysis in neutron flux measurements.