The conventional ultrasonic method for the R-zone of composite has a series of problems, such as reducing detection sensitivity caused by the incident sound beam not be perpendicular to the surface of the R-zone, difficulty in full coverage detection of the R-zone, and inability to evaluate the size of defects along the curvature direction of the R-zone. To solve the above issues, the ultrasonic phased array method was used to detect the R-zone of composite material, and the sound field distribution law in the R-zone was studied. The influence of the number of virtual probe on the detection sound field was analyzed. Through research, it has been found that the number of array element chips affects the sound field distribution. The more chips there are, the greater the difference in sound field distribution strength, the stronger the maximum sound field intensity, and the closer the position is to the surface, the larger the 6 dB sound beam width; Under the focusing detection method, the maximum sound field intensity is generally higher than that of the non focusing detection , and the 6 dB sound beam width is generally smaller than that of the non focusing sound field; The distribution of sound field generated by virtual probes with different angles of arc array probes is basically consistent, and the maximum sound field intensity of the central group chip is the highest. Through experimental verification, the use of ultrasonic phased array method and the use of arc array probes with reasonable detection parameters can effectively detect artificial defects in flat bottomed holes with different angles, burial depths, and aperture of about 3 mm in the R-zone.