Abstract: In order to use ~(252)Cf source effectively and improve testing sensitivity of the nuclear fuel scanner, appropriate moderation material should be selected carefully, and the process of neutron moderation should be optimized. In the paper, neutron fluence rate distributions in several moderation materials were calculated with Monte-Carlo method. The results of several materials and their combinations are given, the higher slowing-down neutron fluence rate and the higher ratio of ~(235)U to ~(238)U fission possibility simultaneously can be got.