The calculation of transmittance is the core of infrared target passive ranging technology, the accuracy of the measured distance is directly affected by accuracy, while the instrument's resolution directly affects the precision of the spectral signal. In order to study the influence of spectral resolution for transmittance, the calculation method of transmittance and the influence factors of the instrument function in the passive ranging system are analyzed. The experimental system is set up to verify the influence of instrument resolution on passive ranging. Firstly, the calculation method of oxygen A band transmittance is introduced, and the theoretical value is calculated. With a halogen lamp as the light source, the spectral curve under different resolutions was measured by the spectrometer at a certain distance, and the measured atmospheric transmittance was calculated. Then, based on the analysis and comparison of the transmittance model and measured transmittance spectrum, the calculation model was fitted and corrected. For a single spectral line, the measured spectral lines with a different resolution of the instrument differ greatly, while for the spectral line with an average point, the spectral signal with the same wave point is averaged to obtain the average effect so that the measured spectral line almost does not change. Specific resolution model selection should be based on different applications, different precision requirements to choose the appropriate resolution. The experimental results show that resolution greatly influences on the transmittance of a single spectral line. With the decrease of resolution, the less the value of captured spectral information points is, the smaller the correlation coefficient of measured target distance will be. The resolution has little effect on the calculation of the average band transmittance, and the measured target distances under different resolutions almost coincide. The higher the instrument's resolution is, the longer the measurement time will be. When the average transmittance is used to calculate the distance of the measured target, the instrument resolution can be appropriately reduced within the accuracy requirements to greatly improve the measurement speed, achieve real-time measurement, and reduce the cost of system construction. The conclusion can provide a basis for the engineering application of transmittance measurement.