The preparation of polymer nanocomposites combined with high strength, toughness, and high transparencyremains a challenge. Aramid fibers are often used as fiber-reinforced materials for their superior mechanical and thermalproperties, but the weak interfacial force between aramid fibers and matrix polymer limits the application in compositematerials. In this work, aramid nanofibers were prepared by a two-step process, which included deprotonation and acidhydrothermal treatment to obtain better dispersions in general solvents. The hydrothermal aramid nanofibers (HANFs) wereused as reinforcing materials and blended with polyacrylonitrile (PAN) to prepare polyacrylonitrile/aramid nanofibers (PAN/HANFs) composite films with different mass fractions of HANFs. The morphologies of HANFs and the thermal, optical, andmechanical properties of composite films were investigated. Interestingly, when the mass fraction of aramid nanofibers wasless than 1.0 %, the composite films were synchronously strengthened and toughened. When the mass fraction of HANFswas 0.5 %, the tensile strength and toughness of the PAN/HANFs composite film reached 62.04 MPa and 22.56 MJ/m3,which were 74.23 % and 162.31 % higher than the pure PAN film, respectively. Besides, its average transmittance in thevisible light region remained 76.34 %. This work may offer a novel and facile strategy for high transparent reinforcedpolymer composites, which have potential applications in high strength fiber or optical film.