Abstract: The Ar concentration in natural gases ranges from n×10^-4～n×10^-5,while that in the air is 9.3×10^-3,which is in 2 to 3 orders of magnitude higher than that of the former.Reducing and preventing natural gas Ar being contaminated by atmospheric Ar during sampling in the field and detecting in laboratories is a key technique for the accurate measurement of Ar isotope.Presently there are no perfect above-mentioned techniques at home and abroad.We tried and improved the measurement method several tens of times,and finally we set up a new technique which can prevent natural gas Ar being contaminated by atmospheric Ar and obtained the surprising and convincing analytic results of natural gas Ar isotope.The technique includes sampling using high pressure steel vases,injecting samples under the conditions of high vacuum and low seepage rate,reducing background values of the instrument and preventing the fluctuation of the environment of laboratory.