Tunable diode laser absorption spectroscopy as an advanced spectral detection method, and it has been widely used in atmospheric environment monitoring, industrial process control, etc. It can realize high-precision detection of component concentration, temperature, pressure and other parameters in different fields. The technology is divided into direct absorption and wavelength modulation spectroscopy technology, by selecting different objective functions, this paper verifies the feasibility of the fitting method in absorption spectrum technology. Both technologies can achieve accurate inversion of absorption information by constructing a suitable target function in combination with a fitting algorithm. The absorption model is established according to the physical process, and the intensity, absorbance and transmittance of the direct absorption technology, the light intensity and harmonic signal of the wavelength modulation spectroscopy technology are fitted as objects. The accurate calculation of the fitting parameters is realized, the accuracy of the processing method is verified theoretically, the application of the fitting algorithm in the development of testing instruments is expanded, and technical guarantee is provided for the development of various fields.