The utility model relates to a detection apparatus, and specifically relates to an apparatus for detecting the sag of a copper starting sheet. The apparatus comprises positioning crossbeams, a support base and a measuring frame fixed on the support base. The multiple positioning crossbeams are successively fixed on the measuring frame from the top to the bottom; the side surfaces of the multiple positioning crossbeams are disposed in the same plane and the plane is vertical to a horizontal surface; and the upper end of the measuring frame is fixedly connected with a supporting block. The apparatus provided by the utility model is simple in structure and convenient and rapid in measuring, and greatly improves the working efficiency.