The invention discloses a positioning mechanism based on a gas inflow flange of an automobile exhaust manifold. The positioning mechanism based on the gas inflow flange of the automobile exhaust manifold comprises a base; supporting rods and a boss are arranged on the top of the base; a drilling mold plate is arranged on the supporting rods; a plurality of mold plate holes are formed in the top of the drilling mold plate, and are used for allowing drill bits to pass through; an adjusting rod is arranged on the drilling mold plate; a pressing plate is arranged at the bottom end of the adjusting rod; the bottom of the pressing plate is provided with a sliding groove which is internally provided with pressing blocks; the boss is provided with a positioning shaft which is located under the pressing plate; fixing blocks and limiting blocks are further arranged on the top of the base; each fixing block is provided with a movable rod; and each movable rod can push the corresponding limiting block to move. The positioning mechanism based on the gas inflow flange of the automobile exhaust manifold can perform accurate positioning on the flange, and meanwhile it can be avoided that the flange inclines in the clamping process.