Polyimide (PI) composite films with thicknesses of approximately 100 µm were prepared via a sol–gel reaction of 3-aminopropyltrimethoxysilane (APTMS) with poly(amic acid) (PAA) composite solutions using a thermal imidization process. PAA was synthesized by a conventional condensation reaction of two diamines, 3,5-diaminobenzoic acid (DABA), which has a carboxylic acid side group, and 2,2′-bis(trifluoromethyl)benzidine (TFMB), with 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) in N,N-dimethylacetamide (DMAc). The PAA–silica composite solutions were prepared by mixing PAA with carboxylic acid side groups and various amounts of APTMS in a sol–gel process in DMAc using hydrochloric acid as a catalyst. The obtained PI–silica composite films showed relatively good thermal stability, and the thermal stability increased with increasing APTMS content. The optical properties and in-plane coefficient of thermal expansion (CTE) values of the PI–silica composite films were investigated. The CTE of the PI–silica composite films changed from 52.0 to 42.1 ppm/°C as the initial content of APTMS varied. The haze values and yellowness indices of the composite films increased as a function of the APTMS content.