The invention relates to an apparatus for semi-automatically measuring an object size based on digital image processing, and belongs to the technical field of image processing. According to the apparatus, by placing a graduation ruler around a to-be-measured object and photographing or scanning the to-be-measured object, a corresponding digital image is obtained, and thus an actual object size iseffectively connected with pixels of the digital image together; then, through corresponding digital image processing, graduation marks on the graduation ruler in the image are identified; next, through calculating, a minimum graduation represented by taking a pixel as a unit is obtained; and at last, by drawing a line and measuring, the size of the to-be-measured object is calculated automatically. The image can be scaled and a straight line can be drawn for multiple times, so by adopting the apparatus to measure the object size, a measurement result is relatively accurate. In the apparatus provided by the invention, the digital image processing is applied to the field of measurement, so the problem of measurement of an object that cannot be moved and needs to be measured for multiple times (such as a footprint or a handprint at a crime scene and the like) is well solved; and therefore, the apparatus has a certain actual application value.