Abstract: This article puts forward a reliable accurate method of system calibrationfor thermal ionization mass spectrometry of uranium isotope. First of all, the authors recognize that the significant systematic errorresults from fractionation effect and nonlinearity of measuring circuit includingthe collector. The residual error factors are treated as random errors. Thesystematic error could be corrected by means of two correction factors, i. e.fractionation correction factor and nonlinearity correction factor. The described above method had been adopted with NBS SRMs as thecalibration standards, the accuracy could be better than 0.3% in the abund-ance ratio range of 200:1 to 1:200.