For natural gas distribution monitoring, it is very important to measure the CO₂ isotope with high precision. In this paper, the tunable diode laser absorption spectrum (TDLAS) technology was adopted to realize the high precision CO₂ isotope measurement through the absorption spectrum line of ¹³CO₂/¹²CO₂ at 4.3 μm. The measurement system consisted of a mid-infrared interband cascade laser (ICL) operating in a continuous wave mode, a long-path multipass cell (MPGC) and a mid-infrared mercury cadmium telluride (MCT) detector. Aiming at the problem that the intensity of ¹³CO₂ and ¹²CO₂ absorption spectra was affected by the temperature, an MPGC high precision temperature control system was developed. In the experiment, five CO₂ gases of different concentrations were configured to calibrate the measurement system, and the response linearity was up to 0.999 6. The results show that when the integral time is 92 s, the isotope measurement precision is as low as 0.013 9‰, which has practical application value.