The paper proposes a foreground extraction method for X-ray imaging auxiliary system. In the radiographic projection process, patient will put their palm on the receptor board. When the depth histogram of palm image and receptor board image are very similar, traditional method is hard to obtain satisfied segmentation results. Therefore, the paper utilizes the improved maximum filter to distinguish the palm image and receptor board image from the original image. In the experimental results, we compare Otsu method with our proposed method. It is evident that our proposed method can obtain a more complete palm image than the Otsu method.