Abstract: In order to realize the high performance detection of CO₂ gas isotope, a multi-pass gas cell pressure control system with high precision and stability was developed in this paper. In terms of hardware, the pressure sensor was connected to the front and back end of the multi-pass gas cell to measure the inside pressure of multi-pass gas cell. The main controller regulated the proportional valves that were at the front and back end of the multi-pass gas cell via PWM signal, so as to realize the closed loop of pressure control. In terms of software, Ziegier-Nichols engineering setting method was adopted to determine three parameters P, I and D. The results show that the control accuracy is ±0.04 Torr (1 Torr=133.322 Pa) when the pressure of the multi-pass gas cell is 60 Torr. In experiment, the developed pressure control system of multi-pass cell was used to measure the absorption spectra of ¹³CO₂ and ¹²CO₂ gas molecules at 4.3 μm. With the increase of gas pressure from 0.026-0.066 atm(1 atm= 101 325 Pa), the peaks of the absorption spectra of ¹³CO₂ and ¹²CO₂ gas molecular increased with the increase of pressure, the width of the absorption spectra also increased with the increase of pressure. Meanwhile, an infrared gas measuring system was used to measure the CO₂ isotope abundance during two hours. The average isotope abundance of CO₂ is −9.081‰, and the fluctuation of measured values is between −8.351‰ and −9.736‰, with the maximum deviation of 0.73‰. It can be proved that the system provides reliable guarantee for high performance detection of infrared CO₂ gas isotope.