The invention provides a forming method of a C-shaped rubber core mold. The forming method comprises the following steps: a first layer of C-shaped unvulcanized rubber is paved in a concave part of a rubber core mold forming mold and tightly bonded with the concave part, and the C-shaped unvulcanized rubber is compacted by a vacuum bag; a core mold sandwich layer is paved on the first layer of C-shaped unvulcanized rubber, and the vacuum bag is adopted to compact the core mold sandwich layer 11 into the C-shaped unvulcanized rubber; the C-shaped unvulcanized rubber paved with the core mold sandwich layer in the previous step is packed as a whole, is fed in a tank for curing, and is demolded to form the C-shaped rubber core mold. The forming method aims to optimize a forming technology of a composite material I-shaped component, provides a reliable core mold to replace a traditional metal core mold and a rubber soft mold, solves the problem of pressure transfer consistency in the forming process, and improves the interior quality of a composite material part.