Abstract: In response to the need for monitoring and testing of the neutron absorption performance of the spent fuel pool grid material in the nuclear power plant reactor during the production and use process, we have developed the spent fuel pool grid frame B₄C_Al material neutron absorption material detection equipment. In order to reduce the influence of the superthermal neutron background in the detection process, we consider using a magnesium oxide thermal neutron filter to filter out the superthermal neutrons. The neutron transmittance and macroscopic total cross-section of 10 and 5 cm magnesium oxide single crystals were calculated theoretically, and the neutron composition on the surface of the moderator was calculated by Monte Carlo simulation and experimental measurements were carried out. The experimental results show that the neutron transmittance of 10 cm magnesium oxide to the ²⁵²Cf neutron source moderated by 8 cm polyethylene is 60.16%, and the relative cadmium ratio is increased by 93.85% compared with that without adding 10 cm of magnesium oxide. At room temperature, magnesium oxide single crystal used as the superthermal neutrons filter of the B₄C_Al detection device is feasible.