The invention provides a design method of an ultra-large type final stage long blade forming roller forging mould. The design of the ultra-large type final stage long blade forming roller forging mould includes a first roller forging mould, a second roller forging mould, and a third roller forging mould. The three roller forging moulds are all in closed roller forging mould designs. The whole forming roller forging process is characterized in that a main change happens in a roller forging forming process in the directions of mould cavity length and mould cavity thickness. The main effect of the first roller forging mould and the first roller forging mould in the roller forging forming process is that a cross-sectional area is reduced, and in this way, a forming roller forging piece can draw long in the length direction. The main effect of the third roller forging mould in the roller forging forming process is that a roller forging cross-sectional is made to be almost close to the shapeof a blade, that is to say, in the roller forging process, the effect of mould-forging-forming can be achieved. In the roller forging process, the material organization performance is improved, the strength and stiffness of an ultra-large type blade can be improved advantageously; the forming portion is made to reach the final size of the ultra-large type final stage long blade; and a mould-forging station in this place can be canceled.