Tunable diode laser absorption spectroscopy technology is widely used in fields such as atmospheric environment monitoring, combustion flow field diagnosis, industrial process control, human breathing detection, etc. Due to its strong selectivity, high sensitivity, high accuracy and non-invasive measurement. Direct absorption technology and wavelength modulation technology are two different measurement methods of tunable diode laser absorption spectroscopy technology. The direct absorption technology measurement system has a simple structure, relatively easy signal processing, low cost, avoid pre-calibration, and is widely used when the measurement gas is a constant component use. When the direct absorption technique measures the gas concentration, it is first necessary to obtain a baseline signal indicating no absorption from the spectral absorption signal and this process is called baseline fitting. At the same time, the baseline fitting will bring large errors to the measurement results, which is one of the reasons why the direct absorption technique cannot reach the low detection limit. Regarding the issue above, this paper is based on the gradient descent method, taking the baseline, gas concentration, absorption line type, etc. as unknowns, and establishing the mathematics of the laser absorption spectrum signal. The model directly fits the transmission signal, and finally obtains the gas concentration information. This method simultaneously fits the lineshape and the baseline. Compared with the traditional integral area method, it enhances the overall consistency of the fit. It is using distributed feedback laser with center wavelength at 1 580 nm. This method is used to measure CO₂ with actual concentrations of 10%, 12%, 14%, 16%, 18% and 20% in the near-infrared laser absorption spectrum gas concentration detection system. The research results show that the curve fitting variances of the direct fitting method at six concentrations are all less than 1×10^-4, the minimum relative error of the measured concentration is only 0.90%, the maximum relative error is 4.40%, the iteration time is within 4 s, and the calculated detection limit is 0.39%. The average relative errors of the concentration obtained by the direct fitting method and the integral area method are 2.63% and 5.74%, respectively. The direct fitting method is better than the integral area method. The experimental research in this paper verifies the feasibility and accuracy of the gas concentration measurement method based on the gradient descent method to fit the spectral absorption signal directly and provides a new idea for directly absorbing technology.