The invention relates to the technical field of mechanical processing positioning. A positioning mechanism comprises a processing stand, a plurality of movable clamping plates and a plurality of positioning sliding blocks, wherein the movable clamping plates are arranged on the processing stand; each movable clamping plate comprises a cushion block, a gasket, a die supporting cushion block, a first clamping plate, a second clamping plate, a plurality of screws and a plurality of bolts; an inverted T shaped sliding channel is arranged on one plane of each cushion block, and the positioning sliding blocks are mounted into the inverted T shaped sliding channels; one end of each positioning sliding block is connected to the corresponding second clamping plate; the second clamping plates are connected to the first clamping plates which are connected to the cushion blocks; the cushion blocks are connected to the die supporting cushion blocks; and the other plane of each cushion block is connected to the corresponding gasket. According to the positioning mechanism, the positioning sliding blocks replace traditional dovetail positioning blocks; the point positioning mode is replaced by a plane positioning mode, thus the influence of abrasion of the positioning blocks on the positioning accuracy can be avoided, and as a result, the processing quality is improved.