Abstract: In Multiple-Input Multiple-Output (MIMO) imaging applications, the configuration of antenna array is one of the key techniques for system design. It has great effects on imaging quality, parameters estimation and targets detection. The performance of the array is related not only to the topology of the array and the number of array elements, but also the spectrum of the radiated signal. The proper array design is helpful to simplify system structure and signal processing. Compared to the traditional narrowband analysis, the antenna array performs some new properties in ultra-wideband (UWB) conditions, which can be used to thin the array. In this paper, an approach is researched for designing linear MIMO array that transmits ultra-wideband stepped frequency waveform, and the corresponding cross-angle resolution, sidelobe level and grating lobe level are derived. The simulation validates the proposed method.