Abstract: This paper introduced the experimental research on the application of ultrasonic phased array detection in the detection of medium and thick plates. Experiments were conducted at different frequencies to compare the sensitivity of longitudinal wave detection, transverse wave detection, blind spots on the upper and lower surfaces of longitudinal wave detection, and blind spots on the edges of transverse wave detection on medium thick plates. The experimental results showed that ultrasonic phased array detection technology can 100% cover all defects in the center of the steel plate, with high detection accuracy, low false alarm rate, and good repeatability; The blind spot on the upper and lower surfaces of the thickness detection steel plate was ≤1.5 mm, and the minimum blind spot on the edge of the plate was ≤3 mm. The detection speed was fast and the cost was low. It can meet the highest requirements of current plate inspection standards.