This paper proposes an origami horn antenna design based on a graphene assembly film. By folding the graphene assembly film (GAF) to change the size of the horn aperture, the antenna's adjustable gain is achieved. The antenna consists of a coaxial waveguide converter and a GAF-based horn. The designed antenna has good impedance matching in the working frequency band of 8.2-12.4 GHz. In this work, the folded graphene assembly film enables the horn to expand or reduce its radiation aperture so as to achieve adjustable gain of 10 dBi, 15 dBi, and 20 dBi. The designed antenna has the advantages of very light-weight, strong flexibility, excellent portability, and good mechanical stability, which provides solutions for applications in specific outdoor and field communication and sensing.