Abstract The present invention provides an ion source comprising two reaction gas inlet devices, which are respectively arranged at the inlet and outlet of the reaction ionization tube. A new chlorine / bromine isotope mass spectrometer is also disclosed, comprising the ion source described above coupled to a capillary gas chromatography system and a magnetic mass spectrometry system. In addition, a chlorine or bromine isotope ratio analysis method is also disclosed. The sample is ionized by a radio frequency discharge reaction with the help of a reaction gas, and the chlorine or bromine in the sample forms pure chlorine anion or bromine anion under the action of a supplementary reaction gas for quality analysis. In the invention, pure chlorine anion or bromine anion is obtained under the action of radio frequency discharge through introduction and control of reaction gas. A new chlorine / bromine isotope mass spectrometer is formed through the design and adjustment of partial magnetic field of magnetic mass spectrum and Faraday detector, with high accuracy in analysis. It completely solves the interference of isobars in previous isotope tests, which can be applied in many fields such as chemical industry, environmental protection, geology, medicine and so on. Drawings 250 250 210 o o230 220 240 Figure 1 322 321 323 331 Figure 2