Abstract: To address severe energy attenuation and challenges in detecting metal specimens caused by low air density and high viscosity in ultrasonic testing of steel pipelines, a steel plate defect detection method based on oblique incidence of air-coupled ultrasonic waves was proposed. A theoretical model of ultrasonic propagation through the air-pipe wall interface was established to analyze transmission coefficients at various incident angles and determine the optimal detection angle. An air-coupled ultrasonic testing model was then developed in COMSOL. The effects of different defect sizes on detection signals were examined, and characteristic parameters were employed to identify steel plate defects. The test results indicated that: ①The theoretically derived optimal incident angle significantly enhanced detection signal energy and accuracy. ②The received signal amplitude inversely correlated with defect size, decreasing as defect depth or width increased. ③The defect identification method based on signal amplitude changes effectively enabled qualitative evaluation of steel plate defects. This research provides a solid theoretical and experimental foundation for applying air-coupled ultrasonic technology in steel pipeline defect detection.