Abstract: In isotopic abundance analysis using thermal ionization mass spectrometry, tailing effect of strong peaks decreases the abundance sensitivity and leads to a poor accuracy of isotopic ratios. In order to solve the problem, the spectral peak tailing contribution factors were discussed and some measures to avoid or eliminate the strong peak tailing were proposed. The results show that the tailing effect can be eliminated to a degree by improving the system’s vacuum via washing the ionic source and the Faraday collectors, and baking the analytic tube, resulting in the improvement of abundance sensitivity of the mass spectrometer.