Air gap Fabry-Perot (F-P) etalon is a common passive optical device, which, as a filter, has been widely used in optical communications, photoelectric measurement, and optical sensing. Based on the classical multi-beam interference theory, the spectrum of an F-P etalon has periodical Lorentz peaks. However, during the calibration and fitting of the transmission spectrum of a fiber air gap F-P etalon, left-right asymmetry and broadening of the transmission peaks are found. These unexpected phenomena lead to a large fitting error if the classical Lorentz formula is employed. In this study, we firstly analyze the relationship between peak asymmetry and broadening, and then investigate the influence of the divergence angle of a fiber collimator and the assembly error of the etalon. Finally, taking the divergence angle and the incident deflection angle into consideration, we propose a multi-peak superposition fitting formula based on the traditional F-P interference theory. With the new formula, a fitting variance of 0.008 and a coincident accuracy of 0.9998 are achieved for the F-P transmission spectra. Compared with the fitting results of Lorentz lineshape, the fitting accuracy is improved by about 15 times. In conclusion, an accurate fitting method for the transmission spectra of fiber air gap F-P etalons is provided, which has important guiding significance for the design and assembly of the F-P etalons.