Abstract: Aiming at the problem of using ultrasonic to detect the internal defects of underwater bridge structures, the waterproof modification method of ultrasonic detector transducer and the method of detecting internal defects of underwater structures based on ultrasonic technology are proposed in this paper. The statistical method is used to determine the anomalies, and the size of the defects is quantitatively analyzed by combining the acoustic time difference and the geometrical relationship between the measurement points of underwater concrete, while the defect height color mapping surface map is used to show the location and size of the defects. The test results show that the underwater ultrasonic detection method proposed in this paper can effectively detect the internal defects of underwater concrete, and the detected location and size of the defects are basically consistent with the actual situation, which verifies its feasibility for the detection of internal defects of underwater structures of bridges.