Oxygen absorption rate is the core of passive ranging technology calculation which used oxygen A band. The cloudless sky background radiation and blackbody radiation which contained oxygen A absorption bands was researched. The oxygen absorption rate distribution of cloudless sky background was calculated under the different conditions of observation zenith angle, times and solar zenith angle used CART. Then the calculation results was compared with the oxygen absorption rate of blackbody radiation with different observation zenith angle at different distances. The results show that the oxygen absorption rate of blackbody radiation is greater than the oxygen absorption rate of cloudless sky background radiation when the detection distance is more than 3 km. So depending on different viewing conditions to set the oxygen absorption rate threshold will improve the detection probability of the target and reduce the influence of background radiation on passive ranging.