The invention discloses a positioning mechanism for a double-head milling machine. The positioning mechanism is used for positioning a workpiece. The positioning mechanism comprises a base and a pedestal. The pedestal is arranged on the base and is movably connected with the base. A fixing pillar, a stand column and a cross beam are arranged on the pedestal. The fixing pillar is installed at one end of the pedestal, and the stand column is installed at the other end of the pedestal. One end of the cross beam is installed on the fixing pillar and is movably connected with the fixing pillar, and the other end of the cross beam is detachably connected with the stand column. A rotary worktable used for carrying the workpiece is arranged in the middle of the pedestal. A positioning air cylinder used for fixing the workpiece is arranged on the cross beam. The positioning mechanism has the beneficial effects that the workpiece can be automatically rotated, the purpose of double-face machining can be achieved through the structural design of the base, the structure is simple, operation is convenient, the labor intensity of operators can be effectively lowered, the work efficiency of the double-head milling machine is improved, later-period maintaining is facilitated, and meanwhile the positioning precision of the workpiece is also effectively improved.