Abstract: To address the problems of difficult high-altitude access, low inspection efficiency, and insufficient probe alignment consistency in manual ultrasonic testing of high-strength bolts on the nuclear power gantry, an automatic ultrasonic phased array testing method for high-strength bolts based on a vision-guided robotic arm was proposed in this paper. First, an image preprocessing procedure was designed, which included bilateral filtering and coarse region of interest (ROI) localization, hue-saturation-value (HSV) background suppression, valley-bottom adaptive threshold segmentation, connected component filtering, and distance-weighted Sobel gradient enhancement. Second, a dynamic curvature threshold contour correction method was proposed, in which the contour circularity was used as a constraint parameter to adaptively adjust the approximation intensity. By this method, local abnormal boundary disturbances were suppressed while the main geometric features of the nut were preserved, overcoming the deficiency of contour distortion caused by fixed-threshold methods. Finally, the visual results were transformed into robotic arm commands through hand-eye calibration and perspective-n-point (PnP) solving, ensuring that the probe was aligned vertically for normal incidence. To verify the effectiveness of the proposed method, experiments were carried out under simulated actual working conditions of the nuclear power gantry, where rust, coating peeling, and reflective interference were introduced. The results show that a center positioning error of (0.58 ± 0.19) mm is achieved by the proposed method, reaching sub-millimeter positioning accuracy. A contour intersection over union (IoU) of 89.2% is obtained, which is 11.3 percentage points higher than that of the fixed-threshold method. A detection success rate of 94.6% is achieved, which is 9.9 percentage points higher than that of the traditional Otsu plus fixed-threshold method, and a defect detection rate of 89% is obtained. The proposed method exhibits strong real-time performance and excellent robustness. Without relying on a CAD model, an efficient and safe automated technical approach is provided by this method for in-situ nondestructive testing of high-altitude high-strength bolts.