The invention relates to a special triangle measuring tool in teaching, in particular to an improved aluminum--niobium alloy coplanar adjustment triangle measuring tool of a through hole corner of a bearing slideway. A bearing pin is arranged on a slide plate lower panel. A shaft pin bearing segment is arranged on the bearing pin. A shaft pin stair is arranged between the shaft pin bearing segment and the bearing pin. The outer end of the shaft pin bearing segment is the outer end of the bearing segment. Rolling bearing inner rings are fixed on the shaft pin bearing segment. The inner side of each of the rolling bearing inner rings is attached to the shaft pin stair. A shaft clamp spring blocking the outer sides of the rolling bearing inner rings is arranged on a clamp ring groove. Corner sight holes are arranged in the slide plate lower panel. Corner scales are arranged on an edge ruler upper panel. A scale slope is arranged on the inner side of the edge ruler upper panel. Length scales are carved in the scale slope. A bearing slideway is arranged in the edge ruler upper panel. A transition open groove is arranged between the bearing slideway and the edge ruler upper panel. A blind groove space is arranged in the bottom of the bearing slideway. The single end of the bearing slideway is opened. The shaft clamp spring is arranged in the blind groove space. A gap is arranged between the shaft clamp spring and the blind groove space.