Abstract: To study the light spot appearance on X-ray detection image for leak place on welding spot of storage tank, several groups of resistance spot welding simulation experiments were carried out on some specimens with anodic oxidized surface.Similar light spots were observed on the X-ray detection image of the specimens.By means of low magnification observation, metallographic examination, microhardness testing and energy spectrum analysis on the section of welding spot on specimens, it was found that the residual anodic oxidation film on the Al plate led to a large contact resistance between the Al plate and the Cu electrode, resulting in a metal penetration phenomenon.The Cu in electrode diffused to nugget zone forming a Cu-rich Al phase, which produced a light spot on the X-ray detection image.