The invention relates to the technical field of resin base fiber reinforced composite prepreg hot pressing tank molding, in particular to a machining method of a special-shaped hollow structure composite part molding mandrel. The machining method adopts a mandrel molding mold; molding space is formed in the mandrel molding mold, and is used for molding the molding mandrel; and the wall surface ofthe molding space is a molding surface. The machining method comprises the following steps: soft mode blanks are folded on the molding surface; the soft mold blanks and the mandrel molding mold are pressed for tight contacting; the soft mold blanks are cured to form a soft mold; a water soluble material is cast in the soft mold until an inner cavity of the soft mold is totally filled; the water soluble material is cured; and the demolding is performed. The molding core mold obtained through the machining method has the advantages of simple machining, convenience to demold and lower cost.