The skin–rib joint is a very important form in carbon fiber composite panels. Because of the complex in geometric structure, internal layer, defect behavior and orientation of fibers therein, defect characterisation, detection and evaluation have been the technical difficulties and focus in their applications. In this paper, a new method for defect characterization and evaluation is proposed using mono-pulse ultrasonic (MU) wave. The experimental results show that the echo signals resulted from the skin–rib joints have very good time-domain resolution under MU reflection condition. When echo signal is close to one pulse cycle, the ultrasonic surface detection dead zone can reach a single layer thickness (about 0.125 mm). Delamination, disbanding, adhesive layer, resin layer and internal layer variation in the skin–rib joints can be characterized, detected and evaluated. The minimum deviation of detected defect is close to 0.0 mm, and the maximum deviation is about 1.0 mm compared with their designed sizes. The depth positioning deviation of detected defect is less than 0.5 prepreg ply thickness. It provides a very effective ultrasonic characterisation and evaluation method for the composite panels. Good practical application has been obtained.