Abstract: The ultrasonic C-scan testing method is used in detecting bond quality of piston ring set of Al2O3 fiber reinforced aluminum matrix composites. By designing and making simulation comparison sample for piston of Al2O3 fiber reinforced metal matrix composites, the ultrasonic C-scan testing method is studied. It shows that the ultrasonic C-scan technique can not only identify and locate the defect, but also precisely measure the defect size.