Abstract: In order to develop high performance dielectric films for high frequency and high speed communication, and make the films possess with ultra-low dielectric constant under high frequency electric field, a kind of porous fillers SiCOH with low dielectric constant were prepared via sol-gel method with polyether F127 as template. Moreover, fluorinated polyimide (FPI) was prepared with 4,4'-(hexafluoroisopropene) diphthalic anhydride and 4,4'-(hexafluoroisopropylidene) bis (p-phenyleneoxy) dianiline as monomers. SiCOH/FPI composite films were successfully prepared by physical blending and thermal imidization, and their properties were studied. The results show that the addition of porous filler SiCOH give the composite films an ultra-low dielectric constant in high-frequency electric field while also providing it with good thermal and mechanical properties. Among them, the composite film with 2.5wt% SiCOH (SiCOH/FPI-2.5%) has an ultra-low dielectric constant of 1.57 at 10 GHz. The glass transition temperature (T_g) of SiCOH/FPI-2.5% is 250.32℃, and the coefficient of thermal expansion (CTE) value is reduced to 47.8×10⁻⁶/℃. The composite films have excellent mechanical properties, with the tensile strength of SiCOH/FPI-2.5% reaching 111.6 MPa, which is 1.4 times that of pure FPI film. The SiCOH/FPI composite films prepared in this work exhibited ultra-low dielectric constant at high frequency electric fields with low filler additions, as well as excellent thermal stability, dimensional stability, and mechanical properties. Therefore, the composite films offer a completely new solution for developing dielectric films for copper clad laminates.