For the detection of deep-sea natural gas hydrates, it is very important to accurately detect the 13 CO 2 / 12 CO 2 isotope ratio of dissolved gas in seawater. In this paper, a 13 CO 2 / 12 CO 2 isotope ratio sensor is investigated, which uses a tunable diode laser absorption spectroscopy (TDLAS) technique at 4.3 µm. The proposed sensor consists of a mid-infrared interband cascade laser (ICL) operating in continuous wave mode, a long optical path multi-pass gas cell (MPGC) of 24 m, and a mid-infrared mercury cadmium telluride (MCT) detector. Aiming at the problem of the strong absorption intensity of the two absorption lines of 13 CO 2 and 12 CO 2 being affected by temperature, a high-precision temperature control system for the MPGC was fabricated. Five different concentrations of CO 2 gas were configured to calibrate the sensor, and the response linearity could reach 0.9992 for 12 CO 2 and 0.9996 for 13 CO 2 . The data show that the carbon isotope measurement precision was assessed to be 0.0139% when the integration time was 92 s and the optical path length was 24 m. The sensor is combined with a gas-liquid separator to detect the 13 CO 2 / 12 CO 2 isotope ratio of CO 2 gas extracted from water. Results validate the reported sensor system's potential application in deep-sea natural gas hydrate exploration. Appl. Sci. 2019, 9, 3444 2 of 13 spectrometry [4], chromatography [5], flame ionization [6], etc. However, these detection methods are complex in structure and poor in long-term measurement stability, which are not suitable for detection in a complex seabed environment. Recently, the tunable diode laser absorption spectroscopy (TDLAS) [7][8][9][10][11] technique was applied in many non-contact gas detection applications, such as environmental, industrial, biology, and safety applications, as a consequence of its high sensitivity and high resolution. In 2006, Lau et al. measured the CO 2 isotopic ratio under the conditions of 45 Torr and a light path length of 100.9 m using a distributed feedback laser in the absorption spectrum of 1.6 µm, achieving the precision of ±1.0% at 3000 ppm [12]. In 2008, AERODYNE RESEARCH, Inc. proposed a CO 2 isotope monitor using a mid-infrared absorption spectroscope with optical path lengths up to 76 m to achieve 0.10% precision for δ 13 C at 1 s [13]. In the same year, Tuzson described in situ, continuous, and high-precision isotope ratio measurements of atmospheric CO 2 using a quantum cascade laser-based absorption spectrometer, and achieved isotope ratios of ambient CO 2 with precision of 0.03 at 100 s of integration time [14]. In 2012, Kasyutich illustrated a laser spectrometer based on a continuous-wave thermoelectrically cooled distributed feedback quantum cascade laser at 2308 cm −1 for the measurement of 13 CO 2 / 12 CO 2 isotopic ratio changes in exhaled breath samples. Typical short-term δ 13 C precisions of 1.1% (1 s integration time) and 0.5% (8 to 12 s integration time) were estimated from the Allan variance plots of recorded data [15]. In 2018, a vertical cavit...