The invention provides a measuring instrument for measuring nitrogen isotope. The measuring instrument comprises an elemental analyzer, an eight-way valve, a sample loop, a gas chromatographic column,an open shunt tube and an isotope mass spectrometer, and the elemental analyzer, the eight-way valve, the sample loop, the gas chromatographic column, an interface device with the open shunt tube andthe isotope mass spectrometer are sequentially connected. The detection limit of nitrogen isotope testing can be reduced by the instrument, so that the nitrogen isotope of a sample containing micro-nitrogen is measured in an online continuous flow mode.