An reverse countersink cutter for digital-control processing is adapted to reversely countersink a digital-control machine tool, improving quality of processing. A cam is in holes of a cutter, a postrod is through the cam, a spring is between the cam and the post rod, the positioning pin of the cutter is engaged into the groove of the cam, a cutter rod is in the cam, the post rod is inserted through the hole of a cutter. A sheath is engaged out of the circumference of the cutter rod by screw threads. A nut is used to lock the sheath, a wrist is on the cutter rod, when the cutter header is inthe wrist, the header could rotate around the wrist. There has a groove in the cutter header, the round pin at the top of the post rod is inserted into the grooves on two sides of the cutter header. When the post rod is backward, the cutter header rotates anticlockwise 90 degree. The width of the groove at the top of the post rod is equal to that of the cutter header, the header is inserted into the groove.