The invention discloses a PFA variable-diameter pipe forming die. The forming die comprises an upper die, a first lower die and a second lower die, and the three dies are detachably connected. The upper die comprises an upper die top board, an upward core, which is arranged on the lower part of the upper die top board, and a downward core, which is arranged on the bottom of the upward core and in the same axis of the upward core. The first lower die comprises a first lower die body and a first lower die hole, which is arranged on the first lower die body. The second lower die comprises a second lower die body and a second lower die hole, which is arranged on the second lower die body. After the three dies are connected and butt jointed with each other, the lower part of the upper die top board, the upward core and the downward core are all embedded into a whole die hole, which is formed by the first lower die hole and the second lower die hole after being butt jointed. Three parts are detachably assembled so as to manufacture all-in-one variable-diameter pipe. The method has the advantages of simple structure, convenient disassembly, shortened processing period, and improvement on production efficiency, furthermore, gas in the dies can be fluently discharged from the upper die during the production, so that the yield is improved and the production cost is reduced.