Upper atmospheric detection based on temporarily and spatially mixed modulated detection mode was put forward. The accurate optical path difference versus the incidence angle of a modified Sagnac interferometer was calculated by the ray-tracing method. Theoretically the parameter of the interferometer was calculated and analyzed. Compared with the combination of the same and different material, it can be concluded that different combination of material is more suited for upper atmospheric detection. This interferometer has many distinct advantages, such as steady state, wide field of view, long optical path difference, temporarily and spatially mixed modulated detection mode etc.. The research developed the theory of upper atmospheric detection and it is quite significant to the research of detection technique, detection mode and engineering in both theory and practice.