To achieve automated ultrasonic testing for large composite components of unmanned aerial vehicles, a gantry-type robotic ultrasonic scanning system has been designed. The system integrates a high-precision gantry mobile platform with a multi-degree-of-freedom industrial robot in a unified design. To enhance the system’s performance, path planning and pose compensation algorithms have been developed. The path planning algorithm automatically generates optimal scanning paths based on the geometric shape of the tested components, ensuring that the ultrasonic probe covers the inspection area at the best angle and distance. The pose compensation algorithm, through real-time feedback and adjustment, effectively eliminates the impact of surface irregularities or robotic motion errors, thereby ensuring the accuracy and consistency of the inspection data. Finally, automated scanning experiments were conducted on large composite structural components of UAVs. The experimental results demonstrate that the proposed system is capable of identifying defect thickness, condition, and shape, with a detection accuracy error of only 2 mm for defect location.