The invention relates to a turning mechanism for assembling lines. A bottom plate is arranged on a base of the turning mechanism through supporting plates, and a transverse moving mechanism is arranged on the bottom plate. A longitudinal moving mechanism is arranged on the transverse moving mechanism through a movable supporting plate. The longitudinal moving mechanism is provided with a carrier. Check blocks capable of blocking the carrier are arranged on the rear side of the upper portion of the movable supporting plate. The turning mechanism further comprises a reverse flow preventing mechanism for preventing the carrier from flowing back on the longitudinal moving mechanism. The bottom plate is provided with a track plate which is of the structure high in the middle and low at the two ends. A self-reset positioning pin is installed at the bottom of the movable supporting plate, the positioning pin penetrates the movable supporting plate, and the top of the positioning pin exactly faces a positioning hole in the carrier. A wheel is installed at the bottom of the positioning pin and can move along the track plate. When the transverse moving mechanism is at the initial position, the wheel is located at the lower end part of the track plate, and the top end of the positioning pin is not inserted into the positioning hole; and when the transverse moving mechanism moves to the middle high part of the track plate, the positioning pin is inserted into the positioning hole, and thus the carrier is prevented from sliding in the transverse direction and is locked.