A series of silica aerogel in-situ filled composite foams were prepared by sol-gel process using ultra-light open-cell flexible polyimide foam as matrix.The density of composite foam was adjustable from 10 kg/m³ to 100 kg/m³，the thickness was adjustable from 1 mm to 400 mm，and the maximum macro size could reach 1 m×1 m.The cell structure，thermal insulation performance and thermal performance were systematically characterized，and the thermal insulation mechanism of polyimide foam in-situ filled with silica aerogel was analyzed.The results show that the introduction of silica aerogel can effectively reduce the thermal conductivity of composite foams at room temperature and improve its thermal insulation performance.With the increase of silica aerogel content，the thermal conductivity of the composite foam decreases from 38.8 mW/（m·K） to 19.6 mW/（m·K）.When the hot end temperature is 300 °C，the thermal conductivity of the composite foam is only 61.1 mW/（m·K）.After filling the silica aerogel，the thermal stability of the composite foam is significantly improved， and the thermal weight loss residue is about 80% at 900 °C.