Abstract: Based on a 1 GWth thorium molten salt breeder reactor (TMSBR), uncertainty of k_eff to nuclear data was analyzed using the TRITON and TSUNAMI-3D modules of SCALE. The results show that uncertainty of k_eff to nuclear data increases from 0.49% to 0.55% with burnup time. At the beginning of life (BOL), ²³²Th（n，γ） cross section is the largest contributor to the k_eff uncertainty, which is about 0.35%, followed by the contributions from ²³³U（n，f） and ⁷Li（n，γ） cross sections. With the increase of burnup, the effects of ¹³⁵Xe（n，γ） and ¹⁴³Nd（n，γ） cross sections on the k_eff uncertainty become more increasingly significant. The analysis for sensitivity coefficient shows that the cross sections for ²³²Th（n，γ）, ²³³U（n，f） and ⁷Li（n，γ） have more significant impacts on the k_eff uncertainty, especially at the neutron energy range of 0.02-0.5 eV, which should therefore be improved.