Geosynchronous orbit is a strategic location in space. It contains communications satellites, data relay satellites, electronic reconnaissance satellites, missile early warning satellites and weather satellites. With the continuous expansion of space mission scope, the application of GEO satellites has been extended to new space missions such as space-based situational awareness, acquisition of non-cooperative target characteristics, in-orbit control, rendezvous and proximity, etc. GEO region has gradually become the focus of attention of various countries, and related technologies have also become the forefront of international competition. In order to control this strategic area and ensure its superiority in space, the US military, supported by its military strategy, advanced technology and financial resources, has deployed a large amount of space offensive and defensive equipment. The United States Air Force put forward the Geosynchronous Space Situational Awareness Program (GSSAP). Through close tracking and monitoring of high-value targets, it can master the mission function, configuration, performance indicators, activity rules and other information of each satellite, and understand the intention, process and effect, so as to strengthen the one-way transparency advantage of space posture. Therefore, it is necessary to conduct in-depth research and analysis on the development history, platform condition, orbit characteristics, mission control and overall index of GSSAP satellite.A portable imaging simulation process is built in this paper. GSSAP imaging simulation system includes target and background modeling, photoelectric sensor modeling, scene imaging generator (Fig.8). The event of GSSAP-4 close to Shijian-20 was selected as the research object (Fig.9). The phase angle and relative distance between GSSAP-4 and Shijian-20 satellite were calculated. The parameters of GSSAP electro-optical sensor were selected, with a 500 mm aperture (Tab.4). The imaging effect of GSSAP EO/IR sensor on Shijian-20 satellite was simulated (Fig.10).The sun phase angle between GSSAP and the target was calculated, which is generally kept between 40° and 160° (Fig.7). At this time, the target was located in the down-light observation area of GSSAP, which can effectively avoid the influence of solar stray light on imaging. The sun phase angle and relative distance between GSSAP-4 and Shijian-20 satellite were calculated (Fig.9). The closest distance of them was 9.54 km. The imaging effects of GSSAP-4 were simulated at different distance and sun phase angle (Fig.10-11). At a distance of 10 km, the target body, solar panels and payload can be clearly seen.In this study, the approach imaging mode of GSSAP satellite is analyzed, and the on-orbit operation mode of orbiting imaging and grazing imaging is extracted. The two-lines elements of GSSAP satellites in recent years were studied. Combined with the orbital information of Chinese high-orbit satellites, dozens of potential close-in reconnaissance activities of GSSAP for Chinese high-orbit satellites were found out. Based on the measured data, the whole process of GSSAP-4 close to the Shijian-20 was analyzed, and the relative distance and solar phase angle of the two were calculated. Under the conditions of distance of 10-133 km and solar phase angle of 44.67°-134.37°, the imaging effect of GSSAP electro-optical sensor was simulated. The results show that GSSAP has carried out multiple close-in surveillance of China GEO satellites. When the aperture is 500 mm, Fnumber is 10, pixel pitch is 6.5 μm, pixel number is 1 024 pixel×1 024 pixel, and integration time is 20 ms, high-resolution fine imaging can be carried out on the target, and the details of the target can be clearly seen, which brings a serious threat to the high value assets of GEO in China.