Abstract: Stable bromine isotope has important significance in the identification and evaluation of the source, cause, formation of groundwater and its hydrogeochemical and physical processes. Because the accelerating voltage of positive thermal ionization mass spectrometer (P-TIMS) (Triton) is not adjustable (10 kV), its maximum measured mass is 310 u. In order to determine heavier ions, the power unit of the TIMS (Triton) was modified, and the acceleration voltage was reduced from 10 kV to 8 kV, then the mass range of the instrument was expanded from 310 u to 350 u. The mass calibration curve was established on 8 kV, and the stable isotope Cs₂Br⁺(m/z 345, 347) were successfully measured. The ion beams of ¹³³Cs₂⁷⁹Br⁺(m/z 345) and ¹³³Cs₂⁸¹Br⁺(m/z 347) were simultaneously collected using Faraday central cup and H₁ cup with reducing accelerating voltage to 8 kV by a special accelerating voltage transformation. The results show that the ⁷⁹Br/⁸¹Br ratio is lower when the bromine content is lower than 10 μg, and the results are in agreement with previous reports when the bromine content is more than 10 μg. The static multicollection method has many advantages, such as, high precision (external accuracy lies 0.09‰-0.18‰), small amount of sample loading (10-20 μg Br) and short acquisition time (just 8 min for 100 cycles). Determination of stable bromine isotopic ratios is achieved by thermal ionization mass spectrometer Triton with 8 kV accelerating voltage using static multicollection of Cs₂Br⁺ ions, which provids the possibility for the high precision determination of bromine isotope and also created the conditions for the study of Br isotopic geochemistry of geological samples.