The invention discloses a flaring mechanism for a pipe expander. The flaring mechanism comprises a flaring installing plate connected with a guide rail of the pipe expander and a flaring pipe arranged on the flaring installing plate, wherein a plurality of flaring pipe installing holes are formed on the flaring installing plate, the flaring pipe is arranged in the flaring pipe installing holes, the front end of the flaring pipe is externally provided with a cone-shaped flaring head, an expanding rod of the pipe expander is covered in the flaring pipe from the back, and the expanding rod can axially move relative to the flaring pipe; and the flaring pipe is externally covered with a clamping device, and the clamping device can axially move relative to the flaring pipe. The flaring mechanism is simple in structure, convenient to install, and space-saving since not needing to be lifted up and down; when a pipe expanding working procedure is carried out by the pipe expander, an opening of a U-shaped pipe leans against the front end face of the flaring pipe, the opening of the U-shaped pipe is clamped by the clamp device, and the expanding rod extends out of the flaring pipe to expand the pipe, so that the quality of finished products can be guaranteed; and after the pipe is expanded, the flaring can be carried out without needing to withdraw the expanding rod, so that the operation time and difficulty for realigning the pipe opening can be reduced, and the defective rate can be reduced as well.