In order to achieve the accurate measurement of gas stable isotope, according to the structure of gas cavity and the characteristics of optical elements, a constant temperature and pressure gas isotope measurement experimental system based on tunable diode laser absorption spectroscopy (TDLAS) technology is designed, which can achieve high-precision and stable control of temperature and pressure in the box. The hardware circuit system including high-precision constant-current source, two-channel temperature sampling module, conditioning circuit and pressure control circuit is designed in the system. The corresponding system closed-loop control software is developed to ensure the stability of the temperature and pressure of the optical cavity. Through the experimental verification, the temperature is controlled at 35ºC and the pressure is controlled at 150 mbar in the present work. The measurement results verified the stability of the temperature and pressure system, as follows: the optimal temperature fluctuation control is ±0.005ºC and the pressure stability is ±0.1 mbar. Then, in our experiment, the CH₄ standard gas is introduced to measure carbon isotope (¹²CH₄ and ¹³CH₄). The results clearly show that the control system is sufficiently stable to be used and the δ¹³C value can reach 0.52‰ according to the Allan variance.