In order to eliminate the influence of the location and number of spectral channels on the ranging accuracy of the oxygen absorption attenuation passive ranging technology, we discuss the basic principle of passive ranging technology based on oxygen absorption attenuation, and analyze the spectral line characteristics of oxygen A and B absorption bands. For spectral channels in the shoulder of the absorption band, the Monte Carlo method is used to analyze the influence of the position and the number of spectral channels and degree of fitting polynomial on the accuracy of non-absorption baseline fitting according to the sum of error squares and the correlation between the non-absorption baseline and the ideal baseline. For the spectral channels in the absorption band, the influence of the absorption rates in different spectral channels on the measurement range and the ranging accuracy is analyzed. Considering the real-time and precision of the system comprehensively, we find that each shoulder of the A absorption band has one spectral channel, and single shoulder of the B absorption band has two spectral channels. The location of spectral channels all should be chosen at the end of each shoulder near the absorption band. The spectral channels in the absorption band can be flexibly selected according to the requirements on the measurement rang and ranging accuracy. Therefore, if the complete spectral curve in the ranging waveband cannot be collected in one-time exposition, the monocular multispectral passive ranging system, using the fewer spectral channels and the simplest linear fitting method, will not only ensure the ranging accuracy of the system, but also reduce the time period of filter replacement and software calculation, and thus enhance the real-time performance of data acquisition and calculation.