Abstract: The precise determination of the uranium and plutonium isotopic composition in the simulated 1AF feed solution by solvent extraction/ion exchange separation-thermal ionization mass spectroscopy(TIMS) was studied in this paper. The precise determination of uranium and plutonium isotopic composition was realized by optimization of chemical separations, and minimization of possible interferences, and proper selection of instrument parameters. Under the selected conditions, the uranium and plutonium isotopic composition in the 1AF feed solution were accurately determined. The relative standard deviations of the main isotopes ²³⁵U and ²³⁹Pu are less than 0.05%.