Synthesizing the multiple-channel and multiple-frequency techniques, the multiple-input multiple-output (MIMO) radar can provide a feasible approach for solving the problems in spaceborne GMTI, such as slowly-moving target detection, and blind speed zones reduction. Effective orthogonal waveform design is crucial for MIMO radar implementation. Therefore, based on an analysis of the performance of orthogonal waveform, with the application of GMTI, the optimized basic parameter choice criteria for stepped frequency orthogonal waveform of spaceborne MIMO radar was proposed. Then, the basic principle of combined space, time and frequency adaptive processing was studied. Finally, simulation results were presented to demonstrate the superiority of MIMO radar in improving clutter suppression and GMTI performance.