A measurement system of air coupled ultrasonic field is set up, and the field of 4 types of air coupled ultrasonic probe is detected. Composite specimens with embedded delamination are detected, effect of acoustic field on the detection results is analyzed. The results show that the low frequency and flat probe have high acoustic field energy and poor directivity, while the high frequency and focusing probe has low acoustic field energy, good directivity and high lateral resolution. The latter should be chosen as long as the penetration capability is guaranteed. Detection distance should be determined according to the acoustic field distribution. The detection sensitivity can be improved when the detected material is located in the focal region of the probe.