Spectral interferometry using frequency comb has become a powerful approach to absolute distance measurement. In this paper, we analyze the principle of spectral interferometry in detail. With the consideration of dispersion, pulse chirp and the power ratio of the reference pulse and the measurement pulse, we develop a Gaussian model, which can be used to determine distances. The frequency of the spectral interference fringe is of key importance. The distances can be directly determined by the frequency of the spectral interference fringe through one-step fast Fourier transform with no filters during the data processing. The simulation results show that the maximum deviation is 1.5 nm when the distance is 1.5 mm theoretically. The comb consists of hundreds of thousands of teeth in the spectral domain, and each tooth can be regarded as a cw laser. We propose a method based on the phases of two close modes. The principle is introduced, and the maximum deviation is 8.7 nm with a distance of 1.5 mm while the minimum deviation is 0.3 nm corresponding to distance of 0.5 mm. We theoretically show that the linear pulse chirp can be used for distance measurement. The measurement principle is analyzed, and the simulation shows that the maximum deviation is 5.3 nm when the distance is 1.2 mm.