The invention discloses a mold for seam trimming of a titanium alloy straight pipe and a method for machining seam trimming. The mold and the method are used for solving the technical problem that the quality of the seam trimming of the conventional titanium alloy straight pipe is difficult to guarantee. According to the technical scheme, the mold is characterized in that a clamping plate (3) and a side plate (7) are fixedly connected on a base (1), the side plate (7), a bolt (8) and the clamping plate (3) are matched with a clamping block (6) and are used for clamping an outer mold (5), a core rod (4) is arranged in the center of the outer mold (5), a trapezoid-shaped slot through which a milling cuter passes, is formed in the core rod (4), and the titanium alloy straight pipe (9) to be machined is placed between the outer mold (5) and the core rod (4); and the core rod (4) is used for propping the molded titanium alloy straight pipe open, after the titanium alloy straight pipe (9) is subjected to seam trimming, the bolt (8) is loosened, the clamping block (6) is moved away, the core rod (4) is taken out, as the diameter of the titanium alloy straight pipe is less than that of the core rod, after the titanium alloy straight pipe rebounds, seams are exactly aligned, the orderly seams are beneficial to welding, so that the quality of the seam trimming of the conventional titanium alloy straight pipes can be ensured.