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The factors that affect isotopic measurement of chlorine based on Cs2Cl+ ion by thermal ionization mass spectrometry were studied. Graphite is essential for the emission of Cs2Cl+ ion from CsCl. No Cs2Cl+ ions are detected in the absence of graphite on the filament. The emission of Cs2Cl+ ion and the measured 37Cl/35Cl ratio were affected by different varieties of graphite, the pH value of the sample solution, and impurities in the solution. High-precision isotopic measurement of chlorine based on Cs2Cl+ ion is achieved only by using graphite with a perfect crystal structure. The measured 37Cl/35Cl ratios were much higher, and even the emission of Cs2Cl+ ion becomes impossible when pH values of the sample solution were higher than 6, corresponding to the presence of excess Cs. The measured 37Cl/35Cl ratios were higher when SO4(2-) and NO3- anions were present. The results show that the measured 37Cl/35Cl ratios are weakly related to the amount of chlorine on filament in a range from 1 to 500 microg.