Due to the long decay period, space debris in Geosynchronous Earth Orbit(GEO) continue to accumulate, and threaten the safety of spacecraft, especially the dangerous debris which cannot be observed from ground based instruments. According to the distribution and motion characteristics of debris, a space based terahertz radar for debris approach warning is studied. Radar Cross Section(RCS)，wavelength, detection range and working process of the radar system are studied. Design and simulation results are presented. The radar carried by a geosynchronous earth orbit satellite utilizes an active phased array of the size 250 mm×250 mm，with working frequency of 140 GHz and average power consumption of 190 W. The radar can search and track debris of diameter 6.8 mm-15 cm in the cone of azimuth±15°, pitching±15°, and the range of 1.95-24.12 km, and can warn the spacecraft at least 4-35 s before the collision happens.