Abstract: A356 aluminum alloy is the main raw material for the manufacture of automobile wheel hubs.It is easy to produce internal defects such as pores and shrinkage cavities in the casting cooling process, which need to be detected by X-ray flaw detection equipment.Based on the introduction of wheel hub X-ray testing equipment, a technical scheme of using the background subtraction method to extract defects was proposed.Firstly, the background image was simulated by using rolling guidance filter, and then the simulated image was subtracted from the original image.The binary image obtained was post-processed by mathematical morphology to achieve the accurate extraction of defects.The experimental results show that the proposed technical scheme has strong versatility and good real-time performance, and can be used for the on-line real-time detection of internal defects of aluminum alloy wheel hubs.