Abstract: Ultrasonic C-scan imaging is an adopted method in composite materials. According to the principle of ultrasonic testing and characteristic of acoustic field, the display in depth is studied. A system of C-scan ultrasonic automatic testing is desighed. It includes host computer, singlechip, ultrasonic emission circuit, incept circuit, signal processing and so on. It has high speed, low costs and accurate testing. The expriment results show that the method can detect a stoma of 1 mm diameter in composite materials. It can realize testing flaws in materials.