The invention discloses a rail outline and undulatory wear sensor positioning mechanism for tracking a central line of the top of a steel rail. The positioning mechanism is arranged on a detection vehicle chassis and stably moves along with a train; and four connecting rods of the positioning mechanism are driven through two main cylinders, so that the entire detection mechanism can be lifted or dropped. When not in work, the positioning mechanism is lifted and is fixedly arranged on the detection vehicle chassis; and when in work, a rail is tightly pressed by the positioning mechanism through the cylinder to keep in close contact with the rail face. A rail outline sensor and an undulatory wear sensor are arranged on two sides of a positioning wheel bracket respectively; and when in work, the positioning wheels are closely attached to the rail through a positioning wheel supporting cylinder. According to the positioning mechanism, a synchronous bracket is additionally arranged, so that the problem of back-forth swing during advancing of the front and rear positioning wheels is solved, and the front and rear positioning wheels can stably move along the center of the rail with the detection device.