The utility model discloses a device for measuring pore -forming diameter in advance, including measuring structure, interior pole, outer pole, it includes measuring stick, gauge to measure the structure, measuring stick one end and interior pole swing joint, the other end and gauge one end swing joint, the gauge other end and outer pole swing joint, outer pole is empty core rod, interior pole position is in outer pole, it has two to measure the structure, sets up within in the position of pole for the symmetry of central line. The gauge is fixed with the little iron sheet of arc with the one end that the measuring stick is connected, be welded connection between little iron sheet of arc and the gauge. For articulated between gauge and the measuring stick, for articulated between gauge and the outer pole, for articulated between measuring stick and the interior pole. Interior pole is from the last millimeter scale that is provided with under to. The thickness d=50mm of the little iron sheet of arc. Specification phi 201mm steel pipe is chooseed for use to outer pole, specification phi 181mm steel pipe is chooseed for use to interior pole.