The DFB laser with narrow linewidth and high side-mode suppression is used to develop a set of open TDLAS-WMS CO₂ gas detection device. At present, most of the CO₂ absorption spectra at 1 570 nm are still used. According to the query in HITRAN database, CO₂ absorption intensity at 2 004 nm is stronger than that at 1 570 nm. The CO₂ molecular absorption peak at 2 004 nm was selected as the absorption spectrum.Based on the detection principle of TDLAS technology, the Lorentz linear gas absorption spectrum was obtained through MATLAB simulation. Gas absorption spectrum wavelength modulation technique to obtain a second harmonic and harmonic signals, because even order harmonic signal strength down line center and the largest signal strength weakened along with the increase of harmonic times fast, odd harmonic signals in the spectrum of gas absorption peak amplitude getting minimum value is zero, so generally choose accidentally time, low harmonic detection of gas concentration, Odd harmonic is used to stabilize the frequency of the laser. In the experiment, the amplitude of the second harmonic signal after modulation and demodulation by a phase-locked amplifier is used to detect the gas concentration.The Herriott type gas absorption tank was designed based on the open environment because of the limitation of the widely used Herriott absorption tank to the detection method of microbial exhaled gas. Compared with the closed absorption cell, the open absorption cell reduces the detection time and operation complexity. The optical structure of absorption cell was simulated by using ZEMAX non-sequence mode. After ray tracing, the theoretical optical path could reach 1 350 mm, the actual optical path increased from 50 mm to 300 mm, and the lower limit value of detection concentration was reduced from 1 300 ppmv to 214.28 ppmv, which effectively improved the detection lower limit ability of the system.The experimental device uses STM32F103VET6 master controller, which controls the DDS chip AD9834 to generate high frequency sine wave signal and 12 bit D/A to generate low frequency sawtooth wave signal. The two signals are filtered and superimposed by the adder. The superimposed signals are converted into current signals to drive the DFB laser through the voltage controlled constant current source circuit. At the same time, the temperature control unit adjusts the temperature stability of the laser. The laser beam enters the open gas absorption cell, and the attenuated laser signal is detected by photodetector and collected by TLC2543 chip analog-to-digital conversion. After the signal is collected, it is sent back to the microcontroller for data processing. The processing results are stored in 32F205RGT6 chip and displayed on the upper computer.Through the configuration of CO₂ gas detection of different concentrations, the amplitude of the second harmonic signal shows a good linear relationship with the concentration, and its fitting coefficient is 0.998 39. The concentration of the gas to be measured can be calculated by fitting the linear equation. CO₂ gas at the concentration of 300 ppmv was continuously detected for 30 min, and the measured results ranged from 285 to 315 ppmv, indicating that the experimental device could continuously and stably detect CO₂ gas. It proves that the second harmonic signal is reliable for concentration detection, and also verifies the stability of the detection device to a certain extent. Allan variance analysis was conducted within 300 s with CO₂ of 300 ppmv. With the increase of integration time, Allan variance showed a trend of decreasing first, then stabilizing and then increasing. When the integration time reaches 101.6 s, Allan variance is in a stable state, and the sensitivity of the detection system is 1.512×10^-5.The test results show that the detection device can accurately measure the concentration of CO₂ gas. The detection device was used to study the concentration of CO₂ gas produced by the respiration of Mycobacterium tuberculosis to provide a basis for the diagnosis of tuberculosis. In the field of microbial medical respiratory gas detection, rapid, accurate and stable detection is realized, and the system can be widely used in other open environment gas concentration measurement.