Abstract: Defect signals collected using conventional TOFD (time of flight diffraction) ultrasonic method is often weak in amplitude because of the low energy of diffracted sound field. In order to improve the testing sensitivity, a new ultrasonic TOFD detection method using line-focusing probe is put forward. Based on geometrical acoustics theory, a line-focusing wedge was designed according to the desired refraction angle of the spindle acoustic field and the focus depth. Through the testing of an aluminum alloy block containing artificial defects, the sensitivity of the proposed method was studied. Through the testing of several friction stir welded aluminum alloy pieces, the advantage of the proposed method was verified. The result shows that the testing sensitivity of the line-focusing ultrasound TOFD method is higher than the conventional method. In the collected D-scan image using the proposed method, the contrast between the defect and background image is sharp, and the image edge can also be kept well. The new method has the advantages in the improvement of defect resolution and the increase of defect detection rate, as well as the reduction of residual error rate, which contribute to a great application prospect.