The invention relates to a preparation method of a Ti-6Al-4V alloy welding wire suitable for repair welding of a titanium alloy casting and belongs to the technical field of nonferrous metal material preparation. The preparation method comprises the steps that S1, alloy welding wire raw materials are mixed and pressed into electrodes, and an alloy cast ingot is formed through smelting; S2, homogenizing annealing is conducted; S3, billet forging is conducted; S4, forging is conducted, and flaw detection is conducted through ultrasonic waves; S5, large deformation is conducted once, rolling is conducted twice, and heating and heat preservation are conducted; S6, hot drawing and finishing rolling are conducted, and surface treatment is conducted; S7, temperature-control hot drawing is conducted; S8, annealing is conducted after cold drawing; and S9, sliding block straightening is conducted, straightening and cutting-off are conducted, and a finished product is obtained through stress relief annealing and polishing. The preparation method has the beneficial effects that nonuniformity, caused by quenching, of alloy components and structures in the caisting process is eliminated; and the contaminated surfaces of wire materials are removed in a centerless grinding mechanical mode, surface impurities are removed, further corrosion of the surfaces of the wire materials is prevented during subsequent storage, and the quality of the wire materials is stabilized and improved easily.