Abstract: Fluoride-salt-cooled high-temperature reactor (FHR) as a Gen Ⅳ nuclear energy system has drawn more attention to the safety and economy. The fine neutron flux density distribution of the full-scale FHR is very important to the radiation damage calculation of the material components, the radioactive source term analysis and the radiation shielding design. To solve this problem, the discrete ordinate (S_N) method was used to provide the source biasing and weight window parameters for the Monte Carlo (MC) method, making the Monte Carlo particles evenly distributed in the whole model space. It could effectively reduce the statistical error of neutron flux density distribution calculation. Based on this method, the coupling code SN2MCNP was written and the neutron flux density distribution of FHR space was finely calculated. After comparison and verification, only 30.1% of the results calculated by analog MCNP has relative uncertainties below the required value (＜10%), while 99.6% of the results by SN2MCNP has relative uncertainties below the required value (＜10%) adopting the same calculation time and statistical method.