In ultra-wideband (UWB) radar imaging applications, high level sidelobes have serious impact on the image quality. In this paper, we analyzed the relationship between point spread function (PSF) and spatial spectrum supporting region, and proposed a method based on spatial spectrum segmentation to improve the image quality. The proposed method reduces the sidelobes and remains the main lobe simultaneously. The simulation and real data results validate the effective, robust and uncomplicated method.