Abstract: This paper introduces an imaging and recognition method of pulse echo ultrasonic C-scan automatics technology to test variable thickness laminates structures which are extensively used in composite parts. The principle lies on the setting up threshold of effective analysis interval based on the thickest area of laminate to collect the “position of last edge”, “position of first edge”, “amplitude of any peak” and “amplitude of last edge” signals, and hence to form four pairs of C-scan images. By analyzing the theory of defect identification, the recognizing method of typical defects, such as delamination, disbonding, inclusion and porosity density is concluded. This technology can solve the problem of hard to acquire effective analysis interval caused by bottom wave shifting in pulse echo method. By using the method of pulse echo ultrasonic C-scan to detect the reference block with typical defects and by quantitatively analyzing detection results, the experimental results show that the defects in variable thickness parts can be well found by this method, and the judgment of defects size is reliable.