The novel cascaded optical imaging system designed herein is based on a fore monocentric symmetric objective and a relay lens array, enabling wide-coverage high-resolution aerial imaging. To satisfy ultralow-altitude requirements for precision agriculture monitoring and pesticide application, we propose a cascaded optical imaging system for ultralow-altitude flight focusing on the optimal mid-curved surface shape and location. Considering the light and small load capabilities of unmanned aerial vehicles (UAV), an aspheric surface is integrated into the system to balance the off-axis aberration, reduce the number of required lenses, and shorten the tube length. The designed optical system comprising a UAV-borne ultralow-altitude wide-coverage camera adopting the proposed cascaded optical structure requires a low altitude of 20--80 m, a focal length of 60 mm, an F-number of 3.4, and a field-of-view of 132°. As expected, the proposed system provides a wide ground coverage and excellent imaging performance under ultralow-altitude flight, thus demonstrating its potential for accurate and efficient agricultural situation monitoring of agricultural UAV.