The invention discloses a positioning mechanism for an automobile clamp. The positioning mechanism comprises a supporting plate, a positioning pin, an annular first spring and an extrusion plug. The positioning pin comprises a screw, a movable plug and an extrusion plug. The movable plug is formed by a pair of completely-same half movable plugs arranged on the left and right. The movable plug is in a cylinder shape, and the top face of the movable plug is concaved inwards to form a coaxial first through hole. The upper portion of the first through hole is in a cylinder platform shape with thelarge upper portion and the small lower portion, and the lower portion of the first through hole is in a cylinder shape. The movable plug rightly sleeves the lower portion of the sleeve through the lower portion of the cylinder shape of the movable plug, and the lower end face of the movable plug is closely attached to the upper surface of the supporting plate. The extrusion plug is in a cylinderplatform shape with the big upper portion and the small lower portion, and the top face of the extrusion plug is concaved inwards to form a coaxial first screw hole. When the extrusion plug is screwedto the lower portion of the screw, a pair of half movable plugs are expanded through the extrusion plug. The positioning mechanism has the beneficial effects of being suitable for various sizes, capable of saving labor force, convenient to use and the like, and the positioning mechanism can be widely applied to the technical field of the automobile clamp positioning mechanism.