Abstract: The researched experiment of the limit ion intensity is finished in collecting data of isotope analysis on the MAT261.When ion intensity is more than 1.0 mV, a precise isotope ratio is gained if maintaining the stability of isotope ion current by strictly controlling electric current of vaporized strip and ionization strip. When ion intensity is between 1.0 mV and 0.5 mV, a reliable isotope ratio can be gained if maintaining the stability of isotope ion current by strictly controlling each step, increasing integral time and collecting data degree .When ion intensity is less than 0.5 mV, a accurate measure can not be done.