Abstract: Carbon fiber reinforced resin matrix composites, renowned for their exceptional specific strength, stiffness, and resistance to corrosion, CFRP are extensively utilized in critical industries such as aerospace and automotive. However, defects arising during the manufacturing and usage processes can significantly reduce safety performance. Therefore, effective detection methods to ensure reliability and longevity are desirable. Air-coupled ultrasonic testing is favored for its non-contact nature, free from couplant-needing, and pollution-free feature, is applied in the defect detection in carbon fiber composite materials. In this paper, the air-coupled ultrasonic oblique incidence method is used to detect and investigate the carbon fiber resin matrix composites, and the effective incidence angle range is determined through theoretical analysis and experimental valiation, and the optimal incidence angle is obtained. The results from air-coupled ultrasonic C-scan using oblique incidence are compared with those obtained from conventional vertical incidence scanning methods. The findings demonstrate that the oblique incidence method not only accurately reveals defects with greater precision but also significantly improves the amplitude of the detection signal, and increases the contrast in C-scan detection, which is conducive to defect identification.