Abstract: Space-air based bistatic radar, one kind of space-based radar(SBR),in some extent can solve the problem of high space attenuation of space-based monostatic radar. However, the clutter spectrum of space-air based bistatic radar is more complicated. Based on the geometric model of space-air based bistatic radar, this paper analyzed the clutter geometrical relationship, clutter Doppler property and mathematical model. Then modeling and simulation of space-time clutter spectrum are given under the condition of earth rotation, different geometric configuration and different transmitter-receiver’s position, and compared with that of airborne bistatic radar. The rationality of the clutter model is illustrated by the theory and simulation result, which can provide reference for clutter suppression of space-air based bistatic radar.