For the spectral absorption curve of cavity ring-down spectroscopy (CRDS), this study uses the MATLAB software and the least-squares algorithm to program six different line-shape fitting algorithms (VP, GP, AVP, AGP, SDVP, and SDAVP algorithms) commonly used for gas absorption spectroscopy and designs interface windows for the parameter setting and fitting result display. The absorption spectrum of the carbon dioxide gas with known concentration is obtained using CRDS. The six aforementioned algorithms are used to fit the absorption spectrum, and the fitting residuals of six different line-shape fitting algorithms are compared. The fitting results denote that the fitting curve is almost coincident with the measuring points. The average AVP residual is the largest, which is 3.1748×10 ^-3, whereas GP and AGP denote the minimum average residual of 1.3212×10 ^-3. This study verifies the feasibility of the six aforementioned line-shape fitting algorithms with respect to the fitting of gas absorption spectrum. The AVP's operation time is the shortest(i.e., 1.7516 s), whereas the SDAVP's operation time is the longest (i.e., 389.682 s). The GP's operation time is less than that of AGP. Therefore, the GP is most suitable for performing the CRDS spectral line-shape fitting with respect to the residuals and operation time.