Abstract: Borehole detection of cast-in-place concrete piles is an important step in ensuring pile foundation quality. In this paper, to address the difficulty in evaluating quality of hole due to the sparse measurement points of existing testing equipment, a Borehole detection system for cast-in-place concrete piles based on ultrasonic technology is designed. The detection probe of the system consists of 12 high-frequency, broadband, and sharply directional ultrasonic transducers that are flat, thin, and small in size, arranged in a layered and dense array. This design improves the accuracy and density of measurement points. Test results show that compared with traditional borehole inspection equipment, the system can collect high-density hole wall contour data and support three-dimensional imaging of the drilled hole morphology, thereby significantly improving the accuracy and intuitiveness of hole evaluation.