In Brillouin lidar system based on the Rayleigh-Brillouin scattering (RBS) and Fabry-Perot (F-P) etalon, the spectral broadening of F-P etalon could produce measurement error of spectrum characteristics. A fitting procedure based on the combination of the dual-parameter fitting and single-parameter fitting for removing the broadening effect is proposed in this paper. Firstly, based on the analysis of the broadening effect of F-P etalon on the spectrum, a dual-parameter Levenberg-Marquardt (L-M) algorithm fitting is proposed and the effects of the initial values of the fitted parameters and the iteration steps in the fitting are analyzed. Then to deal with one parameter’s floating, an extra single-parameter fitting procedure is proposed to improve the measurement accuracy. Experiments show that the fitted results can be stable in the magnitude of MHz and the measurement accuracy of temperature can be improved to 0.1 ℃.