The oxygen A-band passive ranging has become one of the most active research topics in the area of mono-station passive distance estimation. However, the maximum estimating distance with acceptable errors is limited by the saturated zone in oxygen absorption curve. Calculation of the spectral transmittance with Modtran at 1 cm–1 resolution is performed, and through data modeling, a passive ranging formula of zenith angle and average absorption is obtained. This formula avoids saturated zone in the oxygen absorption curve, it makes the effective distance estimation range up to 50 km. In addition, the formula is not sensitive to absorption measurement error. For example, a zero-altitude sensor with a minimum detectable absorbance of Aˉ= 10-2 yields an approximate range error of not exceeding 50 m. The conclusion is expected to break through current situation of passive ranging based on oxygen absorption at short-range, and promote its practical application.