Space based infrared system (SBIRS) is an important part of American′s space defense system, the first of which is called SBIRS GEO-1, in geostationary orbit, launched in May 2011. GEO-1 provides infrared detective and missile warning etc. With high resolution and real-time observing in worldwide. An improved short-axis Schmitt optical structure is designed and simulated based on the study of SBIRS GEO-1 opt-mechanical. Its image quality approaches diffraction limit, with average value of modulation transfer function (MTF) designed spectrum of 0.847 at the Nyquist frequency (14.3 lp/mm). Besides, the deployable outer sunshield servo system is demonstrated, the inhibitory effect of sun stray-light and external thermal flux are analyzed in different time. The results can meet system design requirements. The GEO-1 study provides a certain reference for the infrared imaging system development of our space exploration.