Large quantities of work have been done previously on mineralogical properties and elements extraction of tantalum ores. However, the occurrence states and carrier minerals of U/Th in refractory slag still have not been studied in detail. The objective of this study was to investigate the tantalum slag, in order to determine the distribution and occurrences of uranium-thorium and their carrier minerals. Various techniques were used to detect the slag for chemical compositions, surface morphology and mineral dissociation. The result indicated that the U content was 1.09 x 103 mg/kg and Th was 4.62 x 103 mg/kg, thus the slag had great recovery prospect. Based on diversified characterizations, U was distributed in rare earth minerals as quantities of insoluble tetravalent forms (e.g., UF4, UO2) and a small amount of hexavalent (e.g., UF6, UO3, UO2F2), similarly Th was present in tetravalent state (e.g., ThF4, ThO2). According to AMICS (Automatic Mineral Characterization System) statistical calculation, the carrier minerals of U/Th were fluocerite, monazite and xenotime. The occurrences of U/Th were mainly in adsorption and isomorphism states in carriers, without monomers. There were high proportion of siderite and pyrolusite in slag, and radionuclides had closely relation to rare earth elements and metals. The carrier minerals were predominantly independent grains, some of which were intergrown to Fe-Mn minerals, the rare earth minerals were also associated and symbiotic with each other. By characterizing the U/Th distribution and the properties of carrier ores, this paper provided a theoretical basis for the process of recovering radionuclides from tantalum slag.