Abstract: Airborne cross-track sparse array radar system for downward-looking three-dimensional (3D) imaging is investigated. The sparse array is distributed along the cross-track direction. To improve the transmitting power, orthogonal frequency division signals are used for Multiple-Input and Multiple-Output (MIMO). The multiple phase center aperture synthesis principle is used to make the sparse array coincident with a full array. The phase centers of different sub-band signals will be in the same position by using the matched filter related with spatial location. The broadband signal is obtained by combining the sub-band signals to improve the range resolution. And the downward-looking 3D image is achieved. To expand the observation swaths, a kind of scanning mode which combines ScanSAR mode and SweepSAR mode is proposed, which can reduce the Pulse Repetition Frequency (PRF). System parameters of the airborne cross-track sparse array radar are analyzed. The simulation results indicate the feasibility of the proposed method.