Spiral scanning mode, where ultrasonic probes relatively rotate forward around a tested bar, is commonly used in an automatic ultrasonic testing system for metal bar quality control. Current nondestructive testing standards for bars with ultrasonic technology specify that whether the bar is rejected or accepted should be made in accordance with the amplitude of the echo from a defect in the bar. The quality information obtained by the abovementioned standard testing procedure is only the echo from the defect, and it is too simple to characterize the defect in detail and accurately. Therefore, inspection using imagery and quantitative nondestructive testing (QNDT) of bar quality is the development trend of bar quality control technology in the future. In achieving QNDT of metal round bars, a synthetic aperture focusing on imaging algorithm in a polar coordinate system (pSAFT) and its surrounding scan-mode time-domain synthetic aperture focusing technology (ST-SAFT) were proposed in accordance with the classical ultrasonic synthetic aperture focusing technology (SAFT) theory on a Cartesian coordinate system. Based on the signal set collected by the spiral scanning automatic detection system, the formula of time-delay superposition imaging expressed by the effective synthetic aperture radian was deduced to image the defects in the bar with a cross-sectional view. By preparing a transverse-hole artificial defect sample for the testing experiment, ultrasonic testing data were processed by ST-SAFT, and then a circular section tomography was imaged. Image edge recognition was used to quantitatively evaluate the sizing ability and positioning resolution of ST-SAFT for defect detection. Experimental results show that the measured value of artificial defects with ST-SAFT is equivalent to the real value; the positioning of the transverse side-drilled hole is accurate, and the imaging resolution is significantly better than that of B-scan. The imaging speed for each circular section can reach the order of milliseconds, which can match the mechanical scanning speed of bar inspection for one circle. Therefore, this technology can be used to improve the technical level of ultrasonic automatic testing equipment for metal round bars and ensure safety of the application of metal round bars.