Radiance model of detection system was built based on background radiation and radiation of high altitude balloon. The radiometric signal electrons in focal plane array produced by high attitude balloon in near space and background radiation were calculated precisely by considering the effects of atmosphere transmission, optical system imaging, detector and detector sampling. Then signal-to-noise ratio was deduced for high attitude balloon in near space. An atmospheric modeling tool, Modtran, was used to model radiance of self-emission, specular background reflection and diffuse background reflection. Radiation characteristic of balloon in complex atmospheric and the influence on optic-electronic equipment′s SNR of specular reflectivity, diffuse reflectivity and integration time were analyzed. The research results indicate that the Visible-Near Infrared Ray(VIS-NIR) (0.6~2.4) detector can be used to detect balloon in sunshine and the Long Wave Infrared Ray(LWIR) (8~12) detector can be used to detect balloon in complex atmospheric. With the integral time of 0.25s, the specular reflectivity of 0.32 and the diffuse reflectivtiy of 0.68, or the integral time of 1s, the specular reflectivity of 0.43 and the diffuse reflectivtiy of 0.57, detection of high altitude balloon can easily been achieved.