The invention discloses a rectangular board diagonal precision detection tool. The rectangular board diagonal precision detection tool comprises a reflection tool (23) and a distance measuring tool (24), which are disposed on the corners of the rectangular board diagonal. The reflection tool comprises two reflection tool vertical plates (5), which are in a right angle arrangement; a reflection tool panel (6), which is horizontally connected with one side of each of the two reflection tool vertical plates (5); a button magnet (1); and a rotatable reflection target (2). The distance measuring tool comprises two distance measuring tool vertical plates (12), which are in a right angle arrangement; a distance measuring tool panel (15), which is horizontally connected with one side of each of the distance measuring tool vertical plates; a twist-on distance measuring tool rotating shaft (19); and a distance measuring device (17). The rectangular board diagonal precision detection tool is advantageous in that detection of large board diagonal length data is realized, and whether the deviation of the rectangular board diagonal length exists is determined, and whether a large board cutting precision condition and the rectangular board is required to be repaired caused by out-of-tolerance precision are determined; on-site detection manpower costs are saved, detection precision is greatly improved, and detection results are stable, and therefore inconsistent measuring results are prevented, and measuring efficiency is greatly improved.