Abstract: To realize the defect imaging of the whole interior of a bar section under circum-scanning, a polar coordinates synthetic aperture focusing technique (PCSAFT) is proposed in this paper. According to athe internal structure and propagation path of tbe beam within the bar, the circumferential time-distance curve of the probe and bar in polar coordinate system is derived, and the circumferential detection isochronous surface in polar coordinate system is calculated. Imaging results achieved from a bar with a diameter of 40 mm show that the proposed method can effectively represent the outline of the bar’s outer surface and accurately lacate the defects. For the 11 internal flat bottom holes with a diameter of 2 mm, the maximum angle deviation is less than 3.50°, and the maximum depth deviation of the defects is less than 1.1 mm. Compared with traditional SAFT imaging, the results of CSSAFT imaging proposed in this paper are more intuitive and more suitable for defect detection and determination in automatic inspection processes.