The invention discloses a special fixture for a square hole of a milling open spanner, which is convenient in clamping and comprises a fixture bottom plate fixedly arranged on a worktable; a fixture tie plate is fixedly arranged on the fixture bottom plate; a left locating plate and a right locating plate are fixedly arranged on fixture tie plate respectively; a head part locating chamber and a handle locating chamber are arranged between a left locating plate and a right locating plate; the head part locating chamber is communicated with the handle locating chamber; the head part locating chamber and the handle locating chamber match with the head and handle of the open spanner respectively. The fixture has the advantages that the structure is convenient to clamp the special fixture for the square hole of the milling open spanner; the open spanner can be fixed well for processing the square hole of the open spanner, therefore, the working efficiency can be improved; the operation is simple while the positions of the clamping pieces can be kept invariant without frequent tool setting.