Abstract: Composite of honeycomb sandwich structure has been widely used in the field of aerospace because of its characteristics of high strength,lightweight,sound insulation,heat insulation and other excellent performance.In order to solve the ultrasonic testing problem of this kind of structures,a trajectory planning and gain compensation method has been studied.First,the scanning path is planned to determine the trajectory of the probe.Then the attenuation of the ultrasonic signal due to the variable thickness and surface curvature of the parts is compensated in order to keep the same ultrasonic signal level of different part positions.After that the compensation value of all points should be transmitted to the C imaging module and the C image result can be obtained after scanning.The experimental results show that the defect in variable thickness parts can be well found by this method,which is of very high detection reliability,and easy to implement.