Abstract: Slender-shaft forgings usually have their defects in a direction parallel to the axis. These defects are normally detected by an ultrasonic longitudinal wave straight probe along the forging curved surfaces. As a result of the small diameter of the slender-shaft forging, a tight contact between the curved surface and the probe is difficult to realize, which could lead to different thicknesses of the coupling layer. Thus, a lower ultrasonic transmission coefficient is produced and the ultrasonic detection sensitivity is reduced. In order to realize the effective detection of the defects in slender-shaft forgings, the detection principles of the curved element and phased arrays were analyzed theoretically, and the relevant focusing technique was proposed. The ultrasound field and focusing distribution of the conventional probe, curved element and curved-element phased array probe were investigated by CIVA ultrasonic simulation. Additionally, curved specimens with artificial defects for comparison tests were manufactured and relevant inspection experiments were conducted. It was validated that the proposed technique was suitable for slender-shaft forging detection. The proposed technique had better detection sensitivity, positioning accuracy and quantitative accuracy on slender-shaft forgings compared with traditional straight probe and planar phased array probe.