Based on the detection mechanism of low orbit early warning satellites for the midcourse ballistic missiles, the radiation effects of various radiation sources(solar, earth, atmosphere, clouds) on targets in complex environments were considered. The irradiance generated by the background radiation sources of the three wavelength bands on the target surface was calculated, and the self-radiation and reflected radiation models of the warhead target were established. Based on the calculation results of the infrared radiation of the ballistic missile targets, combined with the diffraction effect of the early warning system, the modified model of the Synthetic Signal-to-noise Ratio(SSR) and the detection range were derived. From these two aspects, the ability of low orbit early warning satellites to detect the midcourse ballistic missiles in deep space was analyzed. The results show that the SSR effect of each radiation source on the target imaging in the complex detection environment can not be neglected. It is approximately 1.2 times that of the SSR only considering the target′s own radiation. The diffraction effect of the optical system of the low orbit early warning satellites has a serious influence on the detection capability of the midcourse ballistic missiles. The ratio of the difference between the unmodified SSR and the modified SSR to the unmodified SSR in the 8-9.4 μm, 9.4-10 μm, and 10-14 μm are 41.9%, 36.7%, and 10.4%, respectively. The detection distance changed with the observation angle, and the detection range is the largest when the detection angle is 0°.