The invention relates to a forming tool and a forming method for a circumferential non-closed variable-cross-section cylindrical part. The forming tool is characterized by comprising a first sealing head, a second sealing head, a liquid filling bag, an upper die and a lower die, wherein the first sealing head is provided with a liquid inlet and an annular groove, one end of the liquid filling bag is an open end, the open end is provided with a sealing flange, the open end of the liquid filling bag is arranged on the first sealing head in a sleeving mode and is embedded into the groove of the first sealing head through the sealing flange, and the liquid filling bag is located in the die cavity formed by the first sealing head, the second sealing head, the upper die and the lower die. The forming tool has the advantages that peculiar shaped part forming machining can be simplified, the material and device utilization rate and the forming part machining precision are remarkably improved, and the manufacturing costs are reduced. Blanks used by the forming method can be circumferentially closed tubular products or cylindrical parts and can be also a circumferentially opened tubular blanks having a certain overlapping amount or planar plates, and a machining application range is wide.