Microwave-induced thermoacoustic imaging (MTAI) has been widely used in biomedical science, and has the potential as an auxiliary measure for clinical diagnosis and treatment. Recently, there are increasing interests in using ultrashort microwave-pumped thermoacoustic imaging techniques to obtain high-efficiency, high-resolution images. However, the traditional imaging system can only provide uniform radiation in a relatively small area, which limits their large field of view in clinical applications (such as whole-breast imaging, brain imaging). To address this problem, we propose an ultrashort pulse microwave thermoacoustic imaging device with a large size aperture antenna. The system can provide a microwave radiation area of 40 cm × 27 cm and a uniform imaging view of 14 cm × 14 cm. With 7 cm imaging depth and a 290 μm resolution. The practical feasibility of the system for breast tumor screening is tested in phantoms with different shapes and in an ex vivo human breast tumor which is embedded in the excised breast of an ewe (π × 5 cm × 5 cm). The tumor can be identified with a contrast of about 1:2. The results demonstrate that the dedicated MTAI system with the uniform large field of view, high imaging resolution, and large imaging depth have the potential for clinical routine breast screening.