Aerial Moving Target Indication (AMTI) carried out by the space-based early warning radar has great significance in military applications.However, when confronted with severe range ambiguities and Doppler ambiguities, the conventional Linear Frequency Modulation (LFM) waveform has low data search rate, few coherent pulses, and weak clutter-suppression and anti-jamming capabilities.In this paper, the design requirements of the synthetic wideband waveform for space-based AMTI radar detection at near, middle and far range are researched on the basis of the American project known as L-band Lightweight Space Based Radar (LLSBR).The simulation results of synthetic wideband signal processing show that the synthetic wideband waveform has excellent performance in the detection of high-speed weak aerial targets for space-based AMTI radars.