The exchange flux of CO₂ between the ocean and the atmosphere is an important indicator for studying the carbon cycle and the ocean acidification issue. The estimation of CO₂ flux mainly depends on the detection of CO₂ in seawater. As a widely used gas detection technology, Tunable Diode Laser Absorption Spectroscopy (TDLAS) has advantages in environmental adaptability, selectivity and good sensitivity, and thus it is potential in the in-situ detection for dissolved gas in seawater. In order to verify the feasibility of applying TDLAS technology to the in-situ detection for dissolved CO₂ in the seawater, this paper integrates the permeable membrane which realizes gas-water separation with the TDLAS gas detection prototype developed in the laboratory to realize the in-situ detection for CO₂ in seawater. The whole prototype is designed as a sealed cabin. The prototype’s material is aluminum alloy which has good tightness, pressure and corrosion resistance. The laser source uses a DFB laser with a central wavenumber of 4 990 cm^-1, and its wavenumber is scanned from 4 992 to 4 994.5 cm^-1 over time, which can contain two absorption lines of CO₂ at 4 992.51 and 4 993.74 cm^-1. The permeable membrane’s material uses Teflon AF-2400 X with excellent pressure resistance and air permeability. Considering of requirements of high detection sensitivity and fast response rate for the prototype in underwater, the prototype uses a miniaturized Herriott-type multi-reflection cavity with good absorption characteristics. The cavity can achieve an effective optical path of 8 meters and a gas sampling volume of only 24 mL. A calibration experiment was done in the laboratory to reduce the influence of the prototype error on the measured value. Five different concentration (202.8×10^-6, 503×10^-6, 802×10^-6, 1 006×10^-6, 2 019×10^-6) standard CO₂ gases were measured by the prototype. The linear correlation (R²) is 99.95% between measured values, and actual values and the prototype has a maximum relatively error ≤8%. The prototype was used to continuously measure the standard CO₂ gas with a concentration of 802×10^-6 for 30 minutes to evaluate the stability of the prototype for a long time working. The average measured value is 802.6×10^-6, and the fluctuation range is only 10×10^-6. The accuracy of the prototype is about 0.5%, which can fulfill the requirement of in-situ dissolved gas detection in seawater. A test was done in an offshore pier with a depth of 3 meters. The prototype successfully obtained the typical absorption spectrum and concentration results of dissolved CO₂ in water for the whole 24 hours. The test verifies the long-lasting stability and robustness of the prototype. Through a shipborne adaptability test of five areas with different depths in the East China Sea, the typical absorption spectrum of dissolved CO₂ was successfully obtained, which proved the adaptability of the TDLAS detection prototype combined with permeable membrane degassing technology within 30 meters depth of seawater.