The expansion machine belongs to the equipment for shaping straight seam metal welded pipes. It uses a conical expansion head to expand inside the steel pipe, eliminating the forming pressure and welding stress of the steel pipe, and ensuring that the true diameter of the entire length of the straight seam welded steel pipe is consistent. The equipment consists of a small car, an expander, a working sleeve, a fixed seat, an oil cylinder, a lubrication station, a platform, a hydraulic station, and an electronic control system. The expander is installed on the trolley and is connected to the oil cylinder by a working sleeve. The expander is composed of an expansion head, expansion block, guide plate, pull rod, etc. A large-scale equipment that uses segmented mechanical extrusion technology to ensure the sizing of straight seam metal welded pipes and eliminate stress.