Abstract: To develop high-performance composite dielectric films for high-frequency applications, polytetrafluoroethylene (PTFE) and fluorinated polyimide (FPI) were employed as the matrix materials, with aminopropylheptyl-polyhedral oligomeric silsesquioxane (NH₂-POSS) serving as the functional filler. Through a composite process of in-situ polymerization and layer-by-layer coating, the PTFE/POSS-FPI composite films with an ultra-low dielectric constant were successfully fabricated. The synergistic effect arising from the fluorinated structure, the intrinsic molecular porous structure of POSS, and the macroscale porous structure in the core layer of the composite film collectively leads to a reduced dielectric constant. A systematic investigation was conducted on the effects of POSS content on the dielectric properties, thermal stability, and tensile mechanical properties of the composite films. The experimental results demonstrate that at a POSS content of 10 mol%, the composite film achieves a dielectric constant of 1.86 (at 10 GHz), a dielectric loss of 0.02 (at 10 GHz), and a dielectric strength of 185.6 kV/mm. The incorporation of POSS remarkably enhances the thermal stability of the composite film, achieving a 5wt% weight loss temperature of 467.2℃, a glass transition temperature of 357.1℃, and a coefficient of thermal expansion as low as 42.5 ppm/℃. Additionally, the composite film maintains good tensile mechanical properties, achieving a tensile strength of 43.6 MPa, which is 2.9 times that of the pure PTFE film (14.9 MPa). The composite film prepared in this work exhibits excellent comprehensive properties under high-frequency electric fields, offering valuable theoretical insights and experimental references for the development of high-performance PTFE/PI-based composite dielectrics.