The invention relates to method for forming a balloon comprising the steps of: providing a preform (5) having a circumferential wall that encloses an internal space (I) of the preform (5)； pre-heating the preform (5) to a shaping temperature by passing a heated gas or fluid via an inlet (5d) of the preform (5) into said internal space(I)； and (c) shaping the preform (5) into a balloon. Furthermore, the invention relates to a device for conducting the method according to the invention
