In this paper, a clutter model for airborne MIMO radar is established, and a computationally reduced-dimension space-time adaptive processing method-Modified GMB method is presented. The method is used to perform space-time adaptively processing by choosing special transmitting beams, receiving beams and Doppler filters and by utilizing the two-dimensional spatial beam forming and temporal Doppler filtering. At last, the clutter suppression performance is emulated and compared under the conditions of different errors. The simulation results show that the use of this method can significantly reduce the computational load and training cells, and the suppression performance of the method is effective.