Abstract: The neutronics performance of coolant for a pebble bed fluoride salt cooled high temperature reactor (PB-FHR) plays an important role on the cell-homogenized group cross-section, and it is different from the case in high temperature gas-cooled reactor (HTGR). The effect of Flibe on cell-homogenized group cross-section of ²³⁸U, ²³⁵U and ²³²Th was investigated based on the MCNP5 code. It is found that due to Flibe, the cell-homogenized absorption cross-section changes sharply compared with that of the helium coolant in the energy group containing strong resonance peak. The cell-homogenized absorption group cross-section gets bigger for ²³⁸U and ²³²Th, while the cell-homogenized fission group cross-section gets smaller for ²³⁵U after including Flibe in the calculation. The relative change introduced by Flibe increases with the packing factor of fuel and molten salt. It is also found that the fuel temperature and the fuel enrichment will not bring evident effect on the relative change. The results express the influence of Flibe on cell-homogenized absorption cross-section and cell-homogenized fission cross-section shows certain regularity.