Abstract: Objective In view of the low reliability of the existing non-destructive testing methods for defects detection in T-joints structure of high-strength aluminum alloy, the technique ultrasonic TOFD (Time of flight diffraction) is employed and studied. The research work aims to improve the current status of non-destructive testing of weld defects for T-joints and provide effective technical support for its quality evaluation. Methods Experiments on artificial defect detection and welding defect detection are conducted. Based on the experimental results of TOFD A-scan, TOFD D-scan, and TOFD B-scan, propagation characteristics of ultrasonic waves in T-joints are analyzed and position of the defects is calculated. The impact of probe spacing measurement error of defect positions is clarified to reduce the interference of toe signals on the diffracted signals from defects in T-joints. Results The research results indicate that ultrasonic TOFD method can detect defects in web weld toes and flange weld toes from T-joints of high-strength aluminum alloy. In the actual welds, due to irregular shape of weld toe, if spacing between probes is too large, signal wave from weld toe will have a greater impact on judgment and identification of signals, potentially even rendering it impossible to distinguish defect diffraction waves. Conclusion Provided that ultrasonic field can cover the entire weld, probe spacing should be minimized to enhance defect recognizability and improve accuracy of positioning calculations. The average measurement error for artificial defects is 0.5 mm, and the average measurement error for weld defects is 1.5 mm.