The invention belongs to the field of aviation part machining, and particularly relates to a machining method for a slender strip type part. The machining method is characterized by comprising the following steps of a. pre-forming the U-shaped transition shape of the part; b. when in use, positioning a part unfolding blank on a die body through part positioning pins, and positioning a cover plateand the die body through die positioning pins; c. after the installation is finished, carrying out flexible wrapping pressing forming on the U-shaped preformed shape of the part through a rubber bag hydraulic forming machine; d. forming bent edges at two closed corners of the part by adopting a manual forming die; and e. completing the final shape forming of the part and tanking out the part successfully. A forming device for the slender strip type sheet metal part with a C-shaped section is characterized by comprising the die body and a base, wherein the die body comprises the cover plate, the part positioning pins, the die positioning pins and the part pre-formed shape, and the base comprises mushroom pins, the part, a first pellet, a second pellet and a third pellet.