A nonlinear-fitting-deconvolution method is put forward for improving the measurement spectral resolution of gas absorption spectra when the instruments function and the absorption line-shape are previously known. The simulated results and experiments show that the method can obviously enhance the spectral resolution and decrease the experiment noise of absorption spectra by gases in the laboratory. The method was used to the measurement of high-resolution atmospheric absorption-spectra in the White cell and the results are satisfactory, the effect of laser line-width was decreased.