In order to investigate the attenuation effect of clear atmosphere on millimeter wave signals, the gases contributing the most to millimeter wave absorption in the atmosphere were analyzed based on HITRAN 2008 database of atmospheric molecular absorption spectroscopy parameters. A fine calculation model of the absorption of molecular spectral lines in the far wing was obtained by utilizing the Gross dynamic line. Based on this model, the physical property parameters and absorption spectrum of 6 primary absorbing gases including H₂O, O₂, and O₃ were calculated using the Line by Line method. Then, attenuation effects of atmosphere on millimeter wave were analyzed in both homogeneous and inhomogeneous path, respectively. Results calculated by the Line-by-Line method agreed well with the results calculated by the Liebe model method. In addition, compared with 94 GHz test system measurement data, the relative deviation was 3.7%. H₂O and O₂ strongly attenuated the millimeter wave signal in the 75-110 GHz millimeter wave band, accounting for more than 95% of the total gas attenuation. When the millimeter wave signal was transmitted horizontally at about 20 km, the attenuated effect of O₃ on the signal should be considered, and the attenuated effect of CH₄, CO, N₂O was negligible. The research results can provide reference for the engineering application of millimeter wave radar parameter design.