This paper presents a new digital X-ray testing technology for welding joint defects based on half wave height method (HWH) in order to ensure the measurement accuracy of the size of the defects. First, the MTF of the digital X-ray testing system is analyzed and the spatial resolution of the system is confirmed. Furthermore, the typical defects of welding joints are simulated, and the size of these defects is measured using HWH. The results are as follows:(1)based on Rayleigh criterion, the spatial resolution of the X-ray testing system is 4.5 Lp/mm determined by the double wire image quality indicator; (2)the measured sizes of the welding joint defects agree with the actual sizes, and the measuring errors are within 6.3%, thus, the influence of human factor can be avoided using the HWH method, and the accuracy can satisfy the testing requirements of conventional products.