Abstract: The pile foundation is an important part of bridge engineering. The underwater structure of pile foundation has some quality problems such as concrete spalling and damage. The 3D imaging sonar can directly identify and measure the diseases of underwater structure. Based on the 3D imaging sonar, a systematic study of "model test-data processing-engineering application" is carried out, The results show that the post-processing of data can effectively remove the noise, clearly present the outline of the specimens, retain the geometric information of the object, and better identify the defects of the specimen. When the size is less than 5 cm, the error is less than 2 cm. The 3D imaging sonar has a good effect in the appearance integrity detection of the underwater part of bridge pile foundation. It can identify the concrete spalling of pile foundation and measure the spalling thickness. The error is within 3 cm, which meets the needs of engineering detection.