The invention provides a method for determining nitrogen or carbon isotope ratio of N2 or CO2 gas on the basis of an elemental analyzer-stable isotope mass spectrometer combined device. The method comprises the following steps of: continuously introducing inert gas from a gas inlet of an oxidation tube of an elemental analyzer; injecting the to-be-detected gas sample N2 or CO2 through a sampling port of a gas sampling device; causing the to-be-detected gas sample N2 or CO2 together with the inert gas enter into a stable isotope mass spectrometer through a reduction tube of the elemental analyzer; and determining the nitrogen or carbon isotope ratio by the stable isotope mass spectrometer. According to the method provided by the invention, working standard gas N2 and CO2 can be determined and meanwhile the pure natural abundant N2 and CO2 gas sample also can be determined; the required external device is saved when the stable isotope mass spectrometer is used for determining the gas sample; the method provided by the invention is simple, convenient and high in result accuracy.