In order to develop clutter mitigation techniques, clutter simulation and characteristics analysis is very important. By introducing the clutter model of Space Based Radar (SBR) and discussing the earth's rotation effect to Doppler frequency, we get the space-time clutter spectrum of SBR. For illustration clutter characteristics, we plot clutter space-time spectrum and clutter angle-Doppler traces for two pulse repetition frequency. Simulation results show that range ambiguity and Doppler ambiguity degrade the performance of clutter mitigation techniques for SBR.