Abstract: Aiming at the problem of non-destructive testing in composite materials, the air-coupled ultrasonic testing based on the penetrative opposite side detection mode was discussed. The principle of the technique was described. Compared with the traditional water-coupled ultrasonic testing, the aviation important composite material γ-TiAl alloys were researched, the detection signal losses and attenuation of the technique were analyzed and calculated. It was concluded that the reflection loss at the medium interface and the transmission attenuation of ultrasonic in air was the main difficulties for the application of air-coupled ultrasonic testing technique. However, the key problem was the development of new ultrasonic transducer.