A physical model was established for studying near-space limb radiance contrast when the target is detected by space-borne infrared surveillance system. The definition of limb contrast and radiation issues of upper atmosphere of the model was described in detail. The target’s limb contrasts were simulated and analyzed to show upper atmospheric radiation properties’ effects to the target visibility based on non-local thermodynamics equilibrium atmospheric model and assumed detector’s noise. The results indicate that the detector’s noise must be considered for evaluating surveillance feasibility correctly. The contrast at the traditional waveband of the strong absorption by water vapor is better than the often-used ‘atmospheric windows’ for the upper atmospheric limb detection. The complicated contrast characteristics result from the relationship among target inherent radiation, atmospheric absorption and background radiation.