Abstract: In order to solve the measurement technical problem of trace 3He in the mixture of hydrogen isotope, the decomposition ratio for single atomic ion is measured and the ability of discrimination for overlapped peaks of hydrogen isotope is enhanced by mass analyzing technique. The results show that the trace concentration of 3He in the mixture of hydrogen isotope can be measured accurately with relative standard error less than 0.6% by this method when the tritium concentration in the mixture of hydrogen isotopes is larger than 5%. 