The invention belongs to the technical field of nondestructive inspection of a nuclear power station, and particularly discloses an automatic welding seam scanning device for a water chamber locatingplate of a heat exchanger of a nuclear power station. The automatic welding seam scanning device comprises an installation rail and a scanning assembly arranged on the installation rail in a sliding manner, wherein the scanning assembly comprises a framework arranged on the installation rail in a sliding manner, one or more scanning rail segments rotatably connected with the framework and a scanning rail car provided with a welding seam inspection device; and the scanning rail segments are unfolded to form a scanning rail for the movement of the scanning rail car. The scanning rail of the device adopts a folded structure, and the scanning rail can be folded in the installation and disassembling process, thereby reducing the overall size; after entering the water chamber, the scanning railcan be unfolded to form a long rail, thereby widening the scanning range of the device; and the installation and disassembling of the device are performed outside the water chamber, so that personneldo not need to enter the water chamber, thereby reducing the irradiation risk of the personnel.