Abstract:The binary image processing is an important tache in the field of image content analysis, which will significantly affect the results of the image processing. In the paper, a new binary image processing algorithm based on projection operator is purposed to deal with the old documents. The rational threshold of binary image is gained through the design and application of projection operator to the gray level image. The border and stain of document can also be automatically detected by the design and application of projection operator to the binary image. It is proved that the binary image process algorithm based on projection operator is a practical and high-efficient method which can decrease time complexity and space complexity greatly.