Abstract: In order to analyze the spectral characteristics of the reflected sun light by different targets, research the ways to reduce disturbance in passive ranging, using spectrometer as measuring equipment, 200 W tungsten halogen lamp as simulated target, glass plate, aluminum plate and plastic plate as interference target, the spectrum of simulated target and the spectra reflected by interference target are acquired and analyzed. In the experiment, the reflected spectrum of interference target and the radiation spectrum of tungsten halogen lamp are acquired, and oxygen absorption is calculated. The difference of oxygen absorption between mirror reflection interference target, diffuse reflection target, the reflection of background and tungsten halogen lamp is analyzed. The result shows that: at 455 m, the oxygen absorption of reflection of mirror target and diffuse reflection target is twice to triple of tungsten halogen lamp. Hence, at a certain distance, the difference of oxygen absorption can be used as a threshold to distinguish targets, and improve detection probability.