The invention discloses a long and narrow girder honeycomb core part machining method. The method comprises the steps that an ultrasonic machine tool is adopted, a honeycomb core blank is fixedly clamped to a vacuum platform, the upper profile of the part is subjected to rough machining and fine machining through a cutter disk, and the machined surface of the part is expanded to the boundary of the blank; and a wedge groove is formed in the edge of the outline of the upper profile by adopting a dagger through cutting, the part is turned over so that the lower profile of the part can be machined, and the outline of the honeycomb core part is obtained axially through cutting by adopting the dagger along the outline line of the part at a time with the nose of the dagger only exceeding the topof the wedge. According to the long and narrow girder honeycomb core part machining method, rough machining and fine machining are directly conducted on the upper profile and the lower profile of thepart, the machined surface of the part is expanded to the boundary of the blank, the turning positioning area is ensured, and the fixed clamping strength is improved; the wedge groove is formed in the upper profile of the part through cutting, part turning positioning is facilitated, machining interference is prevented, the fixed clamping risk is reduced, pulling up and overcut of the part are avoided, and the machining quality is ensured; and one-time cutting machining is conducted on the outline of the part through the ultrasonic dagger, the material removal rate is high, part deformation is low, and the part machining efficiency is improved.