To improve ground infrared detection, the infrared detecting simulation model of ballistic missile in re-entry is established, considering the aerodynamic heating of missile in the re-entry, the infrared radiance attenuation in atmosphere, and background radiation noise. The SNR system is analyzed with different conditions. The results indicate that by selecting working band, increasing the system aperture and exposing time, improving the optical design, and selecting the cloudless weather for observation will improve the effect of system.